This is something reviewed by the Food and Drug Administration. Most of the plastic bottles we purchase and drink from are made from polyethylene terephthalate, or PET. 

It’s often thought that PET is unsafe to reuse, but in reality this is just hysteria. As long as you clean the bottle before reusing it (just as you should when reusing anything), it is perfectly safe. 

Plastics are generally clean materials, so plastic bottles aren’t any more likely to contain bacteria than any other kinds of packaging. Bacteria thrive in warm, moist environments, so any bottle can be home to bacteria if reused without cleaning first. 

So why is it thought that plastic bottles are unsafe to use? Mainly, because of a hoax email that circulated around the internet. It claimed that dioxins form in plastic bottles when reused, which is simply untrue. 

The fact is there is no scientific basis for the claim that they’re unsafe to reuse, and that it’s really just an urban legend. 

(Source)