DEAR DR. BLONZ: I have heard that harmful changes can occur in water bottle plastic during heating in a microwave. I drink lots of water and reuse plastic water bottles for their portability. : Most commercially packaged plastic water bottles are designed for a single use. Consider also that the narrow mouths of commercially packed water bottles make them challenging to wash and rinse. If, however, you are still planning to reuse plastic water bottles, here are some reasonable strategies -- but these are not a guarantee of safety.