MORE INFO ON POSSIBLE TOXINS

The concerns over the safety of the plastic used in single-use water bottles are refuted by the International Bottled Water Association:
"FDA assesses the migration potential of plastics and the substances with which they are made. FDA allows food-contact plastics for their intended use based on migration and safety data. The clearance process includes stringent requirements for estimating the levels at which such materials may transfer to the diet. FDA's safety criteria require extensive toxicity testing for any substance that may be ingested at more than negligible levels.
This means FDA has affirmatively determined that, when plastics are used as intended in food-contact applications, the nature and amount of substances that may migrate, if any, are safe."
The issue of chemicals leaching into water bottles is discussed at length by the Snopes Urban Legends site, Bottle Royale.
Bacterial Concerns for All Water Bottles: The real culprits for the safety of reusing any water bottle are the bacteria and fungi that can grow in damp or partially full bottles once they have been opened. These bacteria generally come from your hands and mouth, or any dirt that comes in contact with the mouth of the bottle.