Triclosan is not currently known to be harmful to humans and is beneficial as an ingredient in toothpaste.
According to the US Food & Drug Administration (FDA):
Triclosan is not currently known to be hazardous to humans.
They do note however, that testing on animals have shown hormone alteration1 3. Regarding toothpaste specifically, the FDA concluded that it, as an ingredient, helped prevent gingivitis2. In that regard, it can be currently concluded that it is indeed safe and beneficial for use as an ingredient in toothpaste.
Mayo Clinic has a statement from James M. Steckelberg, M.D.:
There currently isn't enough evidence to recommend avoiding use of products that contain triclosan — an ingredient added to certain soaps, cosmetics, clothing, cookware, furniture and toys to reduce or prevent bacterial contamination. Recent studies, however, have raised questions about whether triclosan might be hazardous to human health.
Animal studies have shown that triclosan alters hormone regulation. However, data showing effects in animals don’t always predict effects in humans. Other studies in bacteria have raised the possibility that triclosan contributes to making bacteria resistant to antibiotics.
2For some consumer products, there is clear evidence that triclosan provides a benefit. In 1997, FDA reviewed extensive effectiveness data on triclosan in Colgate Total toothpaste. The evidence showed that triclosan in this product was effective in preventing gingivitis.
3Triclosan [5-chloro-2-(2,4-dichlorophenoxy) phenol; TCS] is a broad-spectrum antimicrobial agent that has become one of the most common additives used in consumer products. As a result, TCS has significantly affected the environment and has been frequently detected in human body fluids. Through a long-term feeding study, we found that TCS enhances hepatocyte proliferation, fibrogenesis, and oxidative stress, which, we believe, can be the driving force for developing advanced liver disease in mice. Indeed, TCS strongly enhances hepatocarcinogenesis after diethylnitrosamine initiation, accelerating hepatocellular carcinoma (HCC) development. Although animal studies require higher chemical concentrations than predicted for human exposure, this study demonstrates that TCS acts as a HCC tumor promoter and that the mechanism of TCS-induced mouse liver pathology may be relevant to humans.