Experts have advised on how long you can safely clean your water bottle without properly cleaning it. If it contains water, it's fine to refill it throughout the day without washing it. However, bacteria from your mouth can multiply rapidly, leading to a buildup of biofilm. Dr. Supriya Rao, a gastroenterologist, recommends washing with soap and water at least once daily, even if you only drink water in it, and if you add sugars or flavoring agents to your gym bottle, it should be cleaned. Gerba, a student at the University of North Carolina, noted that students with contaminated water bottles often get particularly contaminated when they walk around campus with them. He recommended putting your bottle in the dishwasher so hot water helps kill bacteria.