I was told not to do this...or to re use a water bottle.
No, it is not bad. There are many myths that try state that toxins are released when you heat or freeze plastic. In some rare cases the plastic can do so, but not on water bottles. If so, this would be known by everyone and the FDA and other organizations would demand that the manufacturers put labels indicating what people are not supposed to do. Most water bottles have little symbols that say recycle. You can re use it at your own free will, or you can recycle it normally with some organization.
Thank you, this is very helpful. now my health crazy grandma can stop bugging me about it ! LOL