system
Given the following post, respond as if you're commenting on Reddit:
human
Purveyors of science! Is it bad to re-use plastic water bottles, and if so - why?
gpt
In general there is no problem re-using plastic water bottles.

It can be hard to properly sterilise plastic. Sufficient heat to be certain of killing/destroying organic contaminants is likely to damage the plastic. Likewise chemicals used for sterilisation may damage the plastic.

Plastic bottles are frequently not made of a single plastic - they are comprised of layers of plastic that provide a variety of properties. The inner-most layer is designed to be safe in conjunction with human consumables (it won't leach harmful chemicals into the fluid). It may be that 'possibly' harmful chemicals are used within the bottle as a whole but they do not come into contact with the food (water).

Damage to this inner layer (see sterilisation above) can reduce the safety of the bottle (and scratches can provide additional places for germs to lodge).

Without sterilisation (i.e. if not cleaned) there is the usual danger from anything else that is used for food that isn't cleaned (bacterial or fungal growth).