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Why shouldn’t you drink water left overnight?

Drinking water is essential for our health and wellbeing. However, there are some important precautions to take when it comes to drinking water that has been left sitting out overnight or for extended periods of time.

Can you drink water left overnight?

It’s generally not recommended to drink water that has been left sitting out open and unattended overnight or for longer periods of time. This is because there is an increased risk of contamination and bacterial growth in water that sits stagnant at room temperature for many hours.

Some of the potential risks and dangers associated with drinking overnight water include:

  • Bacterial growth – One of the biggest risks is bacterial overgrowth. Bacteria multiply quickly in warm, stagnant water.
  • Contamination – Open containers left overnight can allow dust, insects, pet hair, and other contaminants to get into the water.
  • Off tastes and smells – Stagnant water will start tasting and smelling unpleasant as bacteria grow.
  • Illness – Drinking contaminated and bacteria-laden water can cause gastrointestinal distress, nausea, vomiting, diarrhea.

So while it may seem convenient to drink water from a glass left out overnight, it’s generally best to discard that water and pour yourself a fresh glass in the morning.

Why does water go bad overnight?

There are two primary reasons why water that sits out can go bad and become unsafe to drink overnight:

  1. Bacterial growth – Room temperature water provides an ideal environment for many types of bacteria to rapidly multiply. Bacteria levels can increase to dangerous concentrations overnight.
  2. Contamination – Dust particles, pet dander, insects, or other contaminants can easily get into water left out in cups or glasses.

Let’s look at these risks in more detail:

Bacterial growth

One of the most significant risks with drinking overnight water is the potential for dangerous bacterial overgrowth. With the right conditions, bacteria populations can double every 20 minutes. In as little as 6 to 7 hours, just a few bacteria in your water can multiply to dangerous levels.

Many types of pathogenic bacteria could contaminate your water overnight, including:

  • E. coli
  • Salmonella
  • Shigella
  • Campylobacter
  • Listeria
  • Legionella

These bacteria can cause severe gastrointestinal illness if ingested, with symptoms like nausea, vomiting, diarrhea, fever, and cramps.


Sitting water is also at high risk for physical contamination from microbes, particles, and chemicals. Dust, pet dander, pollen, and insect parts could have landed in the glass. Chemicals may have leached from the materials of the cup or container. Biofilm may have formed along the sides. Any of these contaminants can render the water unsafe.

How long can water sit out before it goes bad?

So how long can you leave water sitting before it starts becoming potentially hazardous to drink? Here are some general guidelines on water freshness:

Duration Freshness and Safety
Up to 6 hours Still relatively safe if covered
6 to 24 hours Greater risk of bacterial growth
Over 24 hours Unsafe due to high bacterial levels

The warmer the temperatures, the quicker bacteria can proliferate. On a hot day, even 4-6 hours on the counter may be enough time for dangerous bacterial overgrowth. So when in doubt, play it safe and discard any water that’s been sitting out overnight or longer.

Can you drink cold water left out overnight?

You may think that leaving water out overnight in the fridge door or in a cold glass might be safer. However, experts still recommend against drinking water left out cold and unattended for long periods for several reasons:

  • Bacteria can still multiply in cold water over time, albeit at a slower rate.
  • A glass or open container can still collect dust and contaminants overnight.
  • Condensation from a cold glass can mix with the water, diluting it over time.
  • Off-tastes from bacterial growth may develop.

The fridge door is also one of the least cold spots in the refrigerator, so bacterial growth is still a risk there. A tightly sealed water bottle kept refrigerated may be safer, but ideally you should start each day with a fresh glass of water.

Can you drink hot water left out overnight?

Is it safer to leave water sitting out overnight if it’s still hot? Not necessarily. Here are some of the potential risks with drinking hot water left out for hours:

  • As the water cools to room temperature, bacteria can begin growing.
  • Many types of dangerous bacteria thrive in warm, wet environments.
  • Overnight contamination is still an issue in open containers.
  • It will likely taste unpleasant from bacterial growth.

Hot water from your tap also has risks if it’s been sitting stagnant overnight. The hot water heater can harbor bacteria like legionella. Flushing your pipes with fresh hot water is recommended each morning.

Is it safe to drink cold water after boiling?

If you boil water to purify and kill any bacteria in it, is it then safe to leave sitting out overnight and drink the next day? Unfortunately, boiled water does not stay sterile and safe to drink for long.

Here are the reasons it’s still best to discard boiled water left sitting out:

  • Bacteria can re-enter from the air, dust, utensils used, etc.
  • Boiling kills bacteria but does not remove all possible contaminants.
  • Boiling and cooling water can concentrate any heavy metals present.
  • Off-tastes can develop as gases dissolve back into the cooled water.

For these reasons, it’s recommended to use boiled water within a few hours for optimal safety and taste. You should boil and cool fresh water each day rather than drinking boiled water left overnight.

Should you let the tap run before drinking overnight water?

Some people think letting the water run from the tap for a minute or so before drinking water that’s been sitting out makes it safer. However, this precaution is not enough to make overnight water safe to drink again.

Here’s why:

  • Running water only rinses away pipe biofilm but does not sanitize the stagnant water.
  • Any bacteria already replicated in the water will not be removed.
  • Contaminants can still remain in the glass/container.
  • Unpleasant tastes or smells may linger.

Flushing your water lines is still recommended each morning, but for optimal safety and taste you should always discard any water left sitting out overnight and start fresh.

Can you get sick from drinking old water?

Yes, you can get sick from drinking water that’s been left sitting out too long and has become contaminated. Here are some of the common illnesses that drinking bad water can cause:

Gastrointestinal illness

Stomach bugs and food poisoning-like symptoms are most common from ingesting bacteria-laden water. Nausea, vomiting, diarrhea, cramps, and fever typically develop within 12-48 hours.


This highly contagious virus causes severe vomiting and diarrhea. Outbreaks often occur from contaminated water.

E. coli infection

E. coli bacteria in water can lead to stomach cramps, bloody diarrhea, and severe complications in some cases.

Hepatitis A

This viral infection of the liver leads to fatigue, nausea, fever, and jaundice. Contaminated water is a common source.

Typhoid Fever

Rare in developed countries, typhoid causes prolonged fever, headache, nausea, rash, and diarrhea from fecal-contaminated water.

To avoid these unpleasant and sometimes serious illnesses, it’s always best not to drink water that’s been sitting open or stagnant for too long.

How long does it take to get sick from bad water?

If you accidentally drink contaminated, bacteria-laden water, how soon might symptoms appear? Here’s an approximate timeline:

  • 30 minutes to 4 hours: Initial mild nausea may start.
  • 2 to 12 hours: Stomach cramps, diarrhea, vomiting can begin.
  • 12 to 36 hours: More severe symptoms set in, potential fever.
  • Over 36 hours: Dehydration and hospitalization may be required for serious cases.

However, the timing and severity can vary based on the type of contaminant, your health, and the amount ingested. Seek medical attention if you become dehydrated or have symptoms beyond 48 hours.

How can you make water left out overnight safe to drink?

While it’s ideal to simply discard and get fresh water after letting it sit out too long, if you need to make questionable water safe again, here are some methods to try:

1. Boil vigorously for 1 minute

Boiling water to a rolling boil for at least 1 minute will kill most types of disease-causing bacteria, viruses, and parasites.

2. Use water disinfectant treatments

Water disinfectants containing chlorine dioxide or iodine can kill bacteria and make water safer to drink. However, they may not remove contaminants.

3. Use a water filter

A filtered water pitcher or faucet filter can remove some contaminants. But they vary in effectiveness and don’t necessarily kill bacteria.

4. UV water purifier

A UV purifying device exposes water to ultraviolet light to destroy microorganisms and make the water safer to consume.

However, for water left sitting out more than 24 hours, the taste may still be unpleasant, so fresh water is best.

Should you drink water left out in a plastic bottle?

It’s generally not recommended to drink from plastic water bottles that have been left out for long periods, even if sealed. Here’s why:

  • Bacteria can still multiply over time, even in a closed bottle.
  • Chemicals from the plastic can leach into the water with prolonged contact.
  • The water can take on a plastic taste from the bottle.

The FDA recommends not reusing disposable plastic water bottles for these reasons. They are intended for one-time use. To be safe, drink from a freshly opened bottle.

Is it bad to drink water sitting in a hot car?

It’s best to avoid drinking from water bottles that have been left in hot vehicles for a prolonged period. The heat can promote faster bacterial growth in the water. Chemicals from the plastic bottle are also more likely to leach into hot water.

One study found that water in a plastic bottle reached unsafe temperatures for consumption after sitting in a hot car for just one hour. The tap water reached 130°F (54°C) after an hour. So for both food safety and chemical exposure reasons, it’s better to discard water left in a hot car and start with a fresh bottle.

Can dehydrated people drink water left out overnight?

We often hear the common advice that it’s crucial to stay hydrated and drink lots of water when you’re sick. However, this refers to drinking safe, fresh water only.

People who are dehydrated from illness should avoid drinking questionable water at all costs. Dehydration stresses the body further and makes one even more vulnerable to waterborne bacteria, viruses, and toxins. Hydrate smartly with bottled or boiled fresh water only.

Is it safe to drink dehumidifier water left out?

Using water collected from your dehumidifier may seem like a convenient way to reuse it. However, dehumidifier water should not be consumed without proper filtration and disinfection first. Here are some of the potential risks:

  • Bacterial growth in stagnant dehumidifier tanks.
  • Contamination from airborne mold, viruses, chemicals.
  • Harmful minerals concentrated from evaporation.
  • Unpleasant tastes and smells.

Additionally, if left sitting in the tank for long periods before drinking, further contamination can occur. It’s best to use fresh filtered or distilled water for consumption instead.


Ultimately, it’s always advisable to err on the side of caution and avoid drinking water that has been sitting out open or unsealed for more than a few hours. The risks of contamination and illness simply outweigh any small convenience of drinking water left out overnight. Whenever possible, pour yourself a fresh glass of water to enjoy each day for improved health, safety, and taste.