Xenical (orlistat) causes colon and breast cancer.
Public Citizen, a consumer watchdog group, has petitioned the FDA to remove Xenical from the market due to fears it can lead to breast and colon cancer. The consumer group is also asking the FDA to refuse approval of Alli, an OTC version of the weight loss drug.
This is based on data that suggest orlistat increases the formation of aberrant crypt foci (ACF) in the colon of rats. ACF are abnormal intestinal folds that are believed to be a precursor to colon cancer. But studies in humans show no evidence of hyperproliferative changes in the colon after treatment with orlistat.
There are also post-marketing reports of breast cancer in women taking orlistat. But the drug is minimally absorbed so critics doubt that orlistat is to blame. People taking orlistat are usually overweight, which already puts them at an increased risk for developing cancer.