Researchers tested water that had been sitting in them for 24 hours, and it was full of chemicals. The report says never drink old water from a reusable, soft plastic water bottle. If you have a reusable plastic water bottle and you ever leave water in there overnight, then keep drinking it the next day – STOP IT! Researchers at the University of Copenhagen analyzed water samples after the water had been in there for 24 hours and it was loaded with chemicals. The scientists found hundreds of substances, including photo-initiators, which can mess with your hormones and potentially cause cancer.