triclosan;

Some reports have suggested that triclosan (which is an active ingredient in many toothpastes) can combine with chlorine in tap water to form chloroform, which the US Environmental Protection Agency classifies as a human carcinogen. An animal study has shown that the chemical may improve hormone regulation, and several other laboratory tests have shown that bacteria may be able to resist triclosan in a way that helps them resist antibiotics as well. [17]