The 2-Hour Rule: Is a bottle safe when it’s been left out too long?

On a message board I frequent, a mother recently asked how many of us obey the “don’t feed formula left out over 2 hours” rule. While most people responded that they did abide by this recommendation, a conversation ensued about why we we’d all been told to do this. The answers varied, from vague concepts about bacteria spreading in room temperature bottles, to warnings that the nutrient content of the formula would decrease as time went on.

This all got me wondering: what is the reason we’re told to be so careful about not letting our babies consume “old” formula? For a snacker like Fearlette is – especially one on $30-a-can Alimentum – this rule is a real drag. My sick little theory was that this rule is all b.s., a combined ploy of the formula manufacturers and breastfeeding pushers who want to make it more costly and aggravating to formula feed. So I started researching… and I found plenty of studies on how formula feeding moms don’t follow safe practices, and articles about how unsafe formula handling can kill babies… but no concrete facts, statistics, or studies about just how room-temperature formula becomes an instrument of morbidity.

Go figure, right?

Anyway, let me share with you the information I could find. An Infant Formula Council document warns that the longer formula is left at room temperature, the more likely it is to become contaminated with a bacteria called Enterobacter sakazakii, which can indeed be lethal for premature infants or those with compromised immune systems for other reasons:


A microorganism of particular concern, and which can survive in powdered infant formula, is Enterobacter sakazakii (E. sakazakii). E. sakazakii is an opportunistic pathogen that poses little risk to healthy, term infants. However, in certain highly vulnerable infants this microorganism can cause serious infection that can present severe and life­threatening conditions, including meningitis. This most commonly occurs in low birth weight and immuno­compromised infants, in whom isolated outbreaks of E. sakazakii infections have been reported in a few hospital settings. Some of these cases have been linked to the improper preparation and storage of reconstituted powdered infant formula. For example, blenders used to reconstitute formula have been shown to harbor bacteria. Because reconstituted powdered infant formula is rich in nutrients and is not sterile, it provides a good medium for microbial growth under certain conditions. For example, prolonged periods of storage or administration of prepared powdered infant formula at room temperature will increase the bacterial load. Thus, proper handling and use of powdered infant formula in the health care setting is an important patient safety issue.

The AAP also warns about the dangers of improper formula handling:


In the United States, infant formulas are available in 3 types: ready-to-feed, liquid concentrate, and powdered. Liquid concentrates and powders require appropriate dilution with clean water. Adding too much water may lead to inadequate intake of calories and nutrients, whereas adding too little water may lead to dehydration, diarrhea, and excessive intake of calories. The safety of liquid concentrate and powdered formulas can be compromised if they are diluted with water of poor quality.1 Unlike ready-to-feed and liquid concentrates, powdered formulas are not sterile when purchased and could contain bacteria. To reduce the risk of infant illness caused by bacteria in powdered formula,3 the World Health Organization urges caregivers to use water no less than 158°F when reconstituting powdered formula.4

I checked the references of both these articles – which were coming from two different organizations with dramatically different biases (the AAP one was written, in part, by an IBCLC) – and it was insanely frustrating, They used other articles as backup for these theories that also offered no stark data. I couldn’t find any documentation of cases where children got sick from formula left out of the fridge too long, or suffered long-term health effects because their parents let them drink a bottle they’d been snacking on for 2 hours and 15 minutes.

In terms of preventing contamination Enterobacter sakazakii (ES), a rather stubborn bacteria which does indeed confer risks to infant formula, I did find a long chain of good studies which examined how this bug can survive even under refrigeration, and can cling to surfaces which makes it a real threat in NICU units. Apparently, the longer formula is left out, the more likely it is to become contaminated. But again… no concrete evidence.

Here is what I would like to know (and gold stars for anyone who can find a study which answers these questions):

1. Many of the studies on ES recommend using breast milk or ready-t0-feed formula, since ES has been found in cans of powdered formula…but then they also discuss other ways the bacteria can spread, like through feeding tubes, etc. Couldn’t this bacteria theoretically infect a baby in the NICU fed through one of these tubes with human milk or RTF formula? If so, shouldn’t this be added to the warning? According to this study, ES can thrive in breastmilk; also, although we are innundated with the message that “formula is not sterile” and therefore unsafe…. neither is expressed breastmilk, for the exact same reasons. I don’t see much information on dangerous bacteria being passed to the millions of women who pump their breastmilk daily, and I think it’s worth mentioning.

2. Have there been any documented cases of an infant becoming sick after consuming formula left out more than 2 hours? Why is the 2 hours the limit? I’m looking specifically for a study which examined harmful bacteria growth on a sample of formula over time…

Now, none of this is to say that formula-related dangers don’t exist. I personally know a few parents who’ve experienced health crises due to improper formula usage, which is why I think formula education is incredibly important. It makes complete sense that we should wash our hands prior to making a bottle, or use safe water (there are PLENTY of documented cases in developing nations of babies dying from formula made with contaminated water). But the no-bottles-after-2-hours rule, in areas with clean water or in cases where purified water is being used, just seems to have a startling absence of evidence to back it up.

It seems that this recommendation is based on pure hypotheticals – bacteria could feasibly grow in formula, and two hours seems like a decent cut-off point. But where is the evidence? If we’re seriously concerned about our babies’ safety, shouldn’t we be demanding some real statistics on this potential threat?



Interestingly, while researching this post, I came across a study done by the company which makes Dr. Brown’s bottles. The study looked at how the type of bottle can effect the nutrient decomposition of both breastmilk and formula:


The study measured the level of loss of vitamins A, C and E during baby bottle feedings. The results suggest that the amount of air within a baby bottle, the bottle’s design, and the impact on vitamin levels warrant closer examination….The bottle study was designed to investigate changes in nutrient levels that might occur during a typical bottle feeding time of 20 minutes, using both expressed human milk and infant formula, in both vented bottles and unvented bottles…

As milk is removed from the bottle by the infant, the milk is replaced by ambient air. Nutrient loss is likely caused by the oxidation of nutrients that takes place as air is introduced into the liquid. The amount of air moving through the milk and into the bottle depends on the bottle type, bottle shape, and bottle size.

The lead researcher on this study also mentioned that storage of both human and formula milk could effect the nutrient levels. She offered the following tips for bottle-feeding moms:

Select bottles that minimize air traveling through the bottle.

• Look for little to no bubbles forming in the milk as the baby feeds.

• Use breastmilk that is as fresh as possible.

• Use small bottles that minimize the amount of air at the top of the milk.

• When using infant formula, make it fresh for every feeding.

• Feed babies with small, frequent feedings.

Although this study was funded by the makers of the same bottle which fared the best in the study (Dr. Brown’s), I still think it’s a superb example of the type of research we should be focusing on. It tells us a specific effect of a specific behavior, and offers suggestions to counteract the negative results of the study. Good, useful, empowering information.

I’m certainly not in the business of giving recommendations, and as I’ve said a million times before, I am not a doctor, nor do I play one on tv. But here’s my personal take on it: I’d be pretty careful about the 2-hour rule if your baby is under 6 months old or immuno-compromised in any way. But after that… I don’t know. The fact is, once your kid is on solids, they are going to be at risk for foodborne illness from all sorts of products, not just infant formula. I haven’t found any research that makes me believe Fearlette is at risk because I let her snack on the same bottle for 3 hours rather than 2.

Not that I’d do that. We’re talking hypotheticals here, remember?

About the Author:

Suzanne Barston is a blogger and author of BOTTLED UP. Fearless Formula Feeder is a blog – and community – dedicated to infant feeding choice, and committed to providing non-judgmental support for all new parents. It exists to protect women from misleading or misrepresented “facts”; essentialist ideals about what mothers should think, feel, or do; government and health authorities who form policy statements based on ambivalent research; and the insidious beast known as Internetus Trolliamus, Mommy Blog Varietal.

Suzanne Barston – who has written posts on Fearless Formula Feeder.


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34 thoughts on “The 2-Hour Rule: Is a bottle safe when it’s been left out too long?

  1. I was told BM could be left out for four hours and some said eight hours. I never felt comfortable with that. So I limited how long it was left over by adding small amounts to the bottle as to not waste my pumped milk. The cans of formula usually all say within 1 hour or at least the formula my daughter has been on. During the newborn stage I freaked when I accidentally fed DD a bottle left out for nearly 5 hours. I dozed off then she cried and I swore it was only a couple of minutes. Thankfully she was fine and I moved the clock so I could see it every time.

  2. Breastmilk can sit out a lot longer than formula. I've left bottles of breastmilk out for as long as 8 hours. I'm paranoid about formula though so I always follow the 2 hour rule, even though it sucks to throw out a mostly full bottle.

  3. Even if the water is pure going into the plastic water bottle, it may not be pure coming out. When certain plastics are exposed to warm temperatures, chemicals from the plastic may leach into the container’s contents. Small amounts of chemicals from polyethylene terephthalate (PET) water bottles such as antimony – a toxin in large doses – can accumulate when water is stored in a hot garage or car trunk.

  4. Our family doctor had told me that after a bottle had been started and saliva was introduced, it needed to be used within an hour, but an untouched bottle of either breastmilk or formula could sit out for up two hours total. From what I understand, saliva is the problem– it introduces bacteria. That's why you're not supposed to keep leftovers of baby food if you've dipped the spoon back into it: the spoon introduces bacteria from the saliva. Our doctor also told me that these “rules” can be loosened quite a bit as the kid gets older and develops a stronger immune system, after about 12 months. I'd check with your family doctors / pediatricians.

  5. I've wondered about this for a while! My SIL always left bottles of pumped milk lying about because “breastmilk is sterile and doesn't have to be refrigerated.” Her kid had tons of stomach flus… maybe perhaps not so much the flu? I always tossed the bottle after about an hour because it would gross me out to drink milk that had been out that long. But that was just my personal gross-meter and no real biological concern.

  6. Two hours honestly seems a long time to leave any dairy product sitting out – I wouldn't want to drink a glass of milk that was sitting on the counter for 2 hours, would you?

    I followed a one hour rule for formula so ended up throwing out a lot of formula – maybe half of what we bought got thrown out. I guess I was a little paranoid, but I didn't want formula to be what made my kids sick. I wouldn't have been able to take the guilt!

    • All I’ve ever know is the one hour rule, and I will happy throw the dollar away if I know it could make my son sick. I couldn’t imagine seeing my son laying helpless in the hospital with a stomach infection, cause I was too cheep to throw out the last oz after feeding. Baby’s should drink formula when they’re hungry NOT when they are just looking to run around with a drink. If your kid wants to snack on a bottle for 2 to 3 hours?! then you need to move her to a sippy cup with water. Nibbling on dairy like that is horrible for her teeth and gums. That would be like you constantly drinking sips of a soda for hours.

  7. Yes, my thought would be the saliva introduced during feeding causing problems. I can't remember what the rules were back when I had babies being formula fed but I do have a 10 yr old on a reconstituted powdered formula and a 6 yr old being gastrostomy tube fed on Neocate Advance. What we always did, even when they were babies was make the day's worth up in a jug and keep it in the back of the fridge so if baby was hungry it was easy to give more, or if you had a middle of the night mishap with dropping a bottle.

    With my older kids now I do the same and stick to the 24 hours in the fridge rule. My 6 yr old formula has a variable hang time depending on who you talk to – formula co says 4 hours max and dietitian says 8 hours max. When he was on 10 hours of feeding overnight what we'd do was do a 4 hour hang with the last of the 24 hours worth of formula and then do a 6 hour hang with freshly made formula figuring it wasn't as 'old' then so could stand a little more!
    I think, esp when you look at the rules generated in the NICU you have to remember not only the vulnerability but also so many other things which can become contaminated – tubing, syringes introduced to the tubing etc which put the risks higher, but also if you are tube feeding in any way then the kid doesn't have a chance to taste it and decide it tastes a little 'off'' so that natural check point is bypasses too.

  8. I had to laugh a little bit at a few of the last researcher's suggestions:
    • Select bottles that minimize air traveling through the bottle–>What bottle is that?
    • Look for little to no bubbles forming in the milk as the baby feeds.–>Also how do you do that? Little boy causes lots of bubbles to form :P
    • Use small bottles that minimize the amount of air at the top of the milk.–>Really? Then I would be running to make more twice–my 11 week old takes about 7 oz now. And he gets very mad if I take the bottle out for longer than a few seconds, even to burp
    • Feed babies with small, frequent feedings.–>This made me laugh the most. If I only gave little boy 3 or 4 oz he would scream his head off.

    Anyway…I always wondered about that rule too, but I thought it was just like leaving regular milk out. I wouldn't drink it after two hours. I only let it stay out an hour max past his feeding if there's any left, but he normally drains the bottle (occassionally he'll leave an ounce).

  9. Great point about “basic perishable food rules”; I hadn't really thought of it that way. On the other hand, I tend to be the kind of person who will let my kid eat a french fry that has fallen on a restaurant table, and Fearlette is often found chewing on FC's Crocs. I have definitely sipped coffee with cream that's been out for 3 hours, so I suppose I wouldn't think much about using a bottle which had been out for 3 hours either. ;)

  10. “World Health Organization urges caregivers to use water no less than 158°F when reconstituting powdered formula.” Well, I failed. I always reconstituted powdered formula with water straight from the filter on the tap. Or, if we were out, I used bottled water. My daughter never took a warmed bottle. She was a good drinker though, and feedings went quick, so we never tested the two-hour formula rule.

  11. The rule isn't specific to formula, so I'd follow it, personally.

    We lost power for an extended time during the hurricane this week, and when it came back on I used http://www.foodsafety.gov/keep/charts/refridg_food.html to determine what we had to throw out of our fridge – for *all* of our safety. The guidelines for perishables across the board seems to be 2 hours at above 40 degrees – and the warmer it is, the faster it will spoil/grow bacteria. It's just not a risk I'm comfortable with – though of course, mileage varies.

    The other part of the problem is the bacteria introduced in the process of drinking – i.e. I'd feel better about feeding a new bottle that had sat at room temp for 2 hours vs. letting them snack on the same one beyond 2 hours. An example of this is in the world of tube-feeding. When you have a kid on continuous feeds for huge chunks of the day (or night), the “hang time” of formula is actually anywhere from 4 up to about 12 hours (depending on specific formula and how careful you want/need to be) because there is no contamination from the act of eating.

    Generally, I'd not want to drink a glass of milk that I'd been sipping on for 2 hours, so I'd not want my baby to drink from a bottle that he'd been snacking on for that long, either.

  12. I followed the one-hour rule (snacker on $30 a can neocate here too, and it was painful to toss it out but paranoid husband). The one thing we were uncertain about was whether refrigeration extended the life of left-out formula – e.g. if baby only had half the bottle, could we put it back in the fridge for the next feed. We tended to do so if we got it back in very quickly after the feed (and thank heavens he likes his bottles cold) but never got a satisfactory answer on safety. Thanks for asking this question.

  13. Our NICU made it clear that expressed breastmilk wasn't sterile, but they did all they could as far as sterile nipples, clean bottles, proper refrigeration, etc. I don't think that is the case everywhere, I think some people say it's ok to leave breastmilk out of the refrigerator for a few hours, but I was told to immediately refrigerate after pumping and toss anything not eaten after an hour outside the fridge.

    As far as formula goes, I've always gone by the directions on the can, which say 1 hour for Similac. Usually it works out OK, there are only a few times that we've warmed a bottle and then she won't take it and we end up throwing it away.

  14. I would never feed my baby a bottle of formula that had been left out for over 2 hours. Unless she was crying. Or hungry. Or I was too tired to make a new one. She's still very much alive and very healthy!

  15. 2 hours? The bottle of formula I feed my baby says 1 hour. This leads to a large feeding bill and a lot of wasted formula. Although I will say that my darling little snob nosed child will reject the bottle on his own after about 40 minutes, except in the middle of the night when he is too tired to know what he is drinking.

  16. when my son was born almost 25 years ago he was in hospital with complications. The staff prepared 24 hours worth of powdered formula and left them at room temperature. My doctor also said this was fine unless using premixed formula. I have 3 adult children and they all came out fine with bottles made from powdered formula left on the counter. I would make up the batch each night, knowing how many bottles I would go through. And my doctor told me sterilizing each time was because our mothers and grandmothers mostly lived on wells and there were no regulations or testing.

  17. I pumped for 11 months with my son and formula-fed for one. From the day he started sleeping through the night at four months (we were lucky!) he has always kept a bottle with him in his crib. He would wake up frequently for a few seconds, start sucking on the bottle, and fall right back asleep. After a an hour or two, the bottle would be empty but he never took a pacifer and only like to suck the bottle nipple. Thankfully he never had a gas issue. If I took the bottle out of the crib, most times he would wake up 20 minutes later and start screaming. The doctors told us this was completely normal and fine to do what we were doing. I didn’t change the habit when I went to formula, but that’s the only time the bottle sits out for more than 2 hours. I almost NEVER threw away breast milk and rarely throw away formula because I give him less at a time. My son is completely healthy and has never had an health issues. I think people are just getting paranoid and I do think companies are out to make money. If neither the AAP nor the Infant Formula Council are saying 1 hour, does it have to be 100% true? Most likely not. A lot of people leave butter or creamer out and it doesn’t go bad right away. Formula just has a nasty smell, it’s most likely not “rotting” after an hour as someone claimed. Each formula company also claims they are the best and we all know they can’t all be 100% right there.

  18. Formula bottles will last a full 24 hours in the fridge and fresh breast milk will last 48 hours, if you put them in right away. Let’s say baby is screaming you making him a formula bottle and he instantly clams down starts to drink his bottle for 5 minutes and passes out or he wants to play rather then eat. Then would be a great time to put it in the fridge. But I want to stress if you been trying to get you baby to eat for the last 30mins and it isn’t happening you can either take a brake and try again before the hour is up or just toss it. Fresh breast milk can sit, closed up with a cap not a nipple, at room temp for a max of 6 hours. If you have just safely warmed a bottle from the fridge of either breast milk or formula you only have one hour then toss it. If your child is still thriving only on formula and is taking more then 45 minutes for feeding I would talk to her doctor, it could be as simple as she doesn’t like the nipple your using?

    To the lady that wrote this blog. Parenting is all trail and error, but why take a chance if you already know the possible outcome is your child getting sick, nothing is worth that.

    • Sarah, I wrote a follow up post admitting I was wrong on this topic and explaining the science/research behind the two hour rule.

      However, I want to correct you on one thing: Your example above would still put your child at risk. Once your baby puts lips to bottle, that bottle is only good for an hour. If a bottle is untouched, it can last 2 hours sitting out at room temperature. But once a baby begins drinking, the bottle must be discarded after an hour – you can’t just put it back in the fridge for later use.

      Also, some babies are “snackers”. I wouldn’t think that the shape of a nipple would cause this sort of behavior – more likely, in that case the baby would simply refuse to eat or you’d see other odd behaviors – milk dribbling out of the mouth, fussiness around feeding, etc. It could be that the nipple flow is too slow, though- definitely worth trying a faster flow if your baby is taking 45 minutes to eat. But I don’t want parents panicking just because they have slow eaters…. some babies are just like that. The best solution in this case is to make smaller batches of formula – make 2oz at a time.

    • Kt,

      Thanks. I think if you read the rest of the post, as well as the follow-up I did on it, you’ll see I’m pretty aware of the reasons why this rule is in place. I was admitting to being fallible, and reflecting that the information on this “rule” was not explained in a clear way – we shouldn’t have to rely on the formula company’s websites to get accurate safety information. My own pediatrician told me it wasn’t a big deal to leave a bottle out for a bit over 2 hours, so obviously this isn’t being imparted properly to the necessary parties. Also, Fearlette has been off formula for a year now- she managed to make it through my piss-poor formula handling unscathed. :)

  19. If you have a preemie or NICU baby and you feel better about following the rule exact … then you should do that! I give my son RTF Similac Soy. He has had a bottle that he started drinking – stopped and then drank more later. I am pretty sure total it was out about 2 hours – he is fine with no problems! Also – I have let the bottle of RTF sit out on the counter at room temp for 3 hours bc I forgot to put it in the fridge after I opened it. Once again – he is fine!! As long as you are in an air conditioned home it will be fine! My personal milk has sat out for hours and went back in the fridge and been fine! And in the morning my half and half sits out from like 5-11 and it has never spoiled. We have never gotten sick from it. Though I think you should be careful when it comes to your infant I think those rules are either just to cover their butt or make more $$$$!!! I think it can sit out 2-3 maybe even 4-5 hours and it would be fine! No matter if it touched saliva or not!!!

  20. I make my babies bottles at room tempature. I use powder formula with room tempature water. I’ve never heated it or put it in fridge. . Should I warm the bottles? I do use bottles that are 2 hours old but that’s the limit. My baby is ten months old and will only eat what I eat. Except her bottles of course. Is this normal? She won’t eat the baby food I make her any more. :(
    My question is how does formula go bad if its not heated or cooled?

  21. Hey, I think your site might be having browser compatibility issues.
    When I look at your blog site in Safari, it looks fine but when opening in Internet Explorer, it
    has some overlapping. I just wanted to give you a quick heads up!
    Other then that, amazing blog! Thanks!

  22. Still no one has given a smart answer. Common guys it’s pretty simple! If you know how much your child is gna eat from let’s say a 6oz bottle. Then make the bottle and store the rest.. Ex. If he/she will only eat 5oz, then make the 6oz, pour out 1oz and stir in the refrigerator so you won’t waste it. Then next feeding you only have to make 4oz, then warm up the 1oz and then pour it into the 4 and it makes 5!

  23. I have given my baby formula that has sat out for way past 2 hours. Even from last night and he is fine. You can get sick from eating raw meat and fish, but people eat sushi and cannibal sandwiches and they are fine.

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