In the morning on day 1 of my multi-day hike I opened up a one liter plastic bottle of water and drank half the contents. I stored the bottle in a loop on the outside of my pack and promptly forgot about it. A number of days later, sweating and thirsty I remembered the water bottle, but I worried that I might get an upset stomach if I drank the water.

Was I right to be worried?

How long can I trust the purity of water in a water bottle after it has been opened and exposed to a person's mouth?

  • 2
    Would you edit your Q to say whether it had been exposed to your mouth or someone else's mouth? If your mouth, they are your germs and unlikely to cause you trouble. If someone else's mouth, yech!
    – ab2
    Commented Apr 30, 2018 at 21:24
  • 2
    @ab2 That is not true. Your own cultures and multiply and can cause illness. Your mouth on a bottle is not going to let them multiply much and there as no food in the water. People kiss all the time and don't get sick.
    – paparazzo
    Commented May 1, 2018 at 1:47
  • What does your nose say about the bottle + its content? Where did the water come from?
    – cbeleites
    Commented May 3, 2018 at 15:59
  • @cbeleites The water has a faintly unpleasant odor but not strongly so. The water was purchased from a local bottling company. It doesn't taste chlorinated.
    – Lumberjack
    Commented May 3, 2018 at 16:01
  • If the smell is unpleasant and other water is available (i.e. it is not the last possibility to save from dying of dehydration), I'd not drink. If it is just a very slight smell, I do consider whether washing/rinsing the bottle it can be used for refilling, if the smell is strong I typically don't do that.
    – cbeleites
    Commented May 3, 2018 at 16:41

4 Answers 4


You are way over thinking it.

Presuming the water in the bottle was properly clean, potable and microbiologically safe to begin with, and you're a normal healthy human, the water in the bottle will be OK virtually indefinitely. A few days of a backpacking trip? No question.

If you have a bunch of sticks, leaf matter, or bugs, etc in the water, you can sterilize it and drink it within a day or two just fine. But if you let it sit and stew you might develop a problem, but if a problem develops it will not mater whether you drank some of it or not first.

Disease causing organisms generally need to be present in their hosts to multiply, or you need to ingest a good number of their spores/cysts. Neither of those conditions are likely to occur in your situation and transform your water from safe to unsafe.

  • 1
    If water sits long enough it will start to stink. Even clean water tanks like in an RV need to be cleaned periodically.
    – paparazzo
    Commented May 1, 2018 at 8:32

Did you backwash? Even if so several days.

I keep a jug by my bed that often goes more than a week.

If bottle was clear and hanging on your pack the the sun would kill bacteria.
wiki solar waster disinfection

  • Yes sir. There must have surely been at least a little backwash.
    – Lumberjack
    Commented Apr 30, 2018 at 18:04
  • The solar purifiers I've looked at took steps to keep the pre and post evaporation water separate, is that just a gimmick?
    – user8348
    Commented Apr 30, 2018 at 23:02
  • @notstoreboughtdirt Distillation and simple UV sterilization are not the same.
    – paparazzo
    Commented May 1, 2018 at 1:44
  • So even the evaporation step is extraneous?
    – user8348
    Commented May 1, 2018 at 15:59
  • @notstoreboughtdirt I don't follow use of words step or extraneous. As I said they are not the same. Solar can power distillation but can have distillation without solar. This is too much to cover in comments and not really what they are for. You might want to post a question.
    – paparazzo
    Commented May 1, 2018 at 16:06

Unless you have a weakened immune system, you're worrying too much. I've been in your situation several times, and never once gotten sick from my own water bottle.

If it didn't make you sick the first time you drank from it, it won't make you sick now.

If you're still worried about it nonetheless, or if you are immunosuppressed for some reason, I would suggest investing in a Steripen.


No, don't worry. At least in the US, and presumably a number of other countries, there are organizations that set the safety standards pretty high. As a result, IMHO, we've become much more suspicious of food that's not dangerous. You can backpack with unrefrigerated foods (like cheese and eggs) for a while, and they're still fine, but the safety folks would have to take deep breaths from a paper bag if they found out. ;) So...don't tell anyone, but I've left water in a bottle for months, drank it, and was fine. Now, I believe I have a pretty sturdy immune system, but I bet you're fine.

  • This comment could be improved by removing the holier-than-thou attitude. Commented May 21, 2018 at 20:01
  • @adonalsium I'm not being holier-than-thou here, or at least, not in my thinking. Maybe it's the way it's written, sorry, so feel free to explain why it seems that way. Commented May 21, 2018 at 20:48

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