More than a dozen drugs including a cancer therapy and a bowel medicine are under early scrutiny for potential side effects, U.S. regulators said in a quarterly list released on Thursday.

The Food and Drug Administration said it was investigating cases of liver failure in patients treated with Pfizer Inc's cancer drug Sutent. The agency also is reviewing gastrointestinal perforation reported with Relistor, a drug from Wyeth and Progenics Pharmaceuticals for treating opioid-induced constipation.

Appearing on the list means that FDA has identified a potential safety issue, but does not mean that FDA has identified a causal relationship between the drug and the listed risk, the agency said.

The FDA releases a quarterly list of safety probes as part of an effort to inform the public about early investigations of potential side effects that have been reported. The list released on Thursday covers issues identified between January and March 2009.

Pfizer said in a statement it would work with the FDA to assess the reports (on Sutent) and determine a path forward.

In clinical trials, liver failure was seen in less than 1 percent of patients with solid tumors who were treated with Sutent, the company said.

We stand behind the safety and efficacy of Sutent, Pfizer said. The drug is approved for treating advanced kidney cancer and a digestive cancer known as gastrointestinal stromal tumor.

On Relistor, Wyeth and Progenics are engaged in discussions with the FDA regarding intestinal perforations that were reported and continue to monitor this issue closely, Wyeth spokeswoman Gwen Fisher said.

The FDA also said it was probing King Pharmaceuticals' Flector pain patch for hypersensitivity reactions and Medicis Pharmaceutical Corp's Solodyn antibiotic for autoimmune disorders in pediatric patients. Solodyn is approved for treating red, pus-filled pimples and bumps from moderate to severe acne.

Officials at King and Medicis could not immediately be reached for comment.