If a reusable water bottle is your go-to for staying hydrated, you’re probably refilling it several times per day—at the gym, while running errands or between work calls. But how often do you actually stop to clean it? We asked a microbiologist and food safety expert about the ins and outs of washing your water bottle—you may need to do it more often than you think.

Why Your Water Bottle Gets Slimy

A peek inside a water bottle that hasn’t been washed in a few days may yield no visible cause for concern, but wipe the interior with your finger or a towel, and you’ll probably feel or see a slimy substance called biofilm. “Formation of a biofilm is a multistep process and allows bacteria to anchor to the surface, where they often create a sticky substrate that covers them,” explains microbiologist and food safety professor Kniiel.

Without a proper wash to rid the water bottle surfaces of this biofilm, bacteria continue to grow and can even detach into the water, says Kniel. In other words, you could be drinking potentially harmful bacteria.

Kniel notes that scientific studies have shown that stainless-steel surfaces tend to have less bacteria-loaded biofilms; bacteria can grow on a range of surfaces, including glass, and may form more quickly on plastic.

The environment, as well as the liquid stored within water bottles, can also play a role in the growth rate of bacteria. “The first thing that I think about when it comes to water bottles and the growth of microorganisms is that microorganisms grow especially well in moist environments [and they will] grow faster in a water bottle stored at room temperature compared to a water bottle stored in the refrigerator,” says Kniel. “If the bottle was containing a liquid with sugar, like juice or a sports drink, the bacteria may grow faster compared to water.” Touching water bottles with dirty hands or sipping from them while you’re sick can also expose water bottles to bacteria.

How to Clean a Reusable Water Bottle

Kniel recommends rinsing water bottles daily with hot water and washing them thoroughly every few days to help reduce bacterial buildup—regardless of how many times you’ve actually used them—as going too long between washes can cause black growth (aka mold). And Kniel says this should be done even more often for water bottles used by young children and those with weak or compromised immune systems due to increased risk of illness.

Some water bottles are easier to clean than others, like glass water bottles that are typically dishwasher safe. Others may have small openings or straws and caps that are more difficult to clean. Either way, Kniel recommends washing water bottles with hot, soapy water to help remove bacteria, making sure to reach all surfaces.

How to Clean Dishwasher-Safe Water Bottles

Step 1: Empty all contents from water bottles.

Step 2: Disassemble each part, including lids, straws and even the silicone gaskets. Place the bottles with the open end down onto a tall tine on either a bottom or top dishwasher rack, depending on your water bottles’ recommended cleaning instructions. Straws and small parts can be placed in utensil baskets.

Step 3: Run a hot cycle with dishwasher soap.

Step 4: Let all parts dry thoroughly, either in the dishwasher or on a drying rack, before replacing parts and storing.

How to Clean Reusable Water Bottles By Hand

Step 1: Empty all contents from water bottles and disassemble each part, including lids, straws and silicone gaskets. Rinse with hot water.

Step 2: Wash all pieces with hot, soapy water.

For the tip: Use a narrow brush to help you reach into bottles, straws and small crevices.

Step 3: Rinse with hot water until the water runs clear and no bubbles are visible.

Step 4: Place all parts upside down on a clean drying rack with proper air flow until completely dry.

How to Deep-Clean Reusable Water Bottles

Water bottles that have come in contact with especially dirty surfaces, that you’ve used while sick, or that haven’t been washed in some time may require a deeper clean.

Step 1: Empty all contents from water bottles.

Step 2: Disassemble each part, including lids, straws and gaskets.

Step 3: Fill bottles with 1:1 ratio of water and vinegar and let it soak overnight. Small pieces can be submerged in a container filled with the same solution.

Step 4: Pour out the solution from the bottles and containers, then scrub each piece thoroughly with a clean brush.

Step 5: Rinse all parts with hot water until the water runs clear and no bubbles are visible, and there are no noticeable odors.

Step 6: Place all parts upside down on a clean drying rack with proper air flow until completely dry.

For the tip: Invest in a drying rack designed for water bottles to increase airflow and help prevent bottles and their additional parts from tipping over.

The Bottom Line

Reusable water bottles—especially those constructed of plastic or containing small parts—can be a breeding ground for bacteria. A food safety expert recommends rinsing them daily and performing a deeper clean every two to three days. Cleaning them in a dishwasher cycle, or using hot, soapy water, can help remove the bacterial biofilm. For more stubborn buildup, use a mixture of baking soda and vinegar. Ensuring that all parts are dry before reassembling or storing is key to keeping them clean between uses.