Diet drug rimonabant (Acomplia / Zimulti) may have a new worrisome side-effect, according to M.D. Anderson Cancer Center researchers. A study conducted in mice suggests that rimonabant may increase the number and size of polyps that are a precursor of colorectal cancer.

Reporting in the August 1st edition of the journal Cancer Research, the scientists said their preclinical research showed that the cannabinoid cell receptor CB1 -- associated with relieving pain and nausea, elevating mood and stimulating appetite -- also appears to play a tumor-suppressing role.

Acomplia, which induces weight-loss by blocking the CB1 receptors both in the peripheral and central nervous system, is available in Europe and a variety of other countries, but has been kept from the U.S. market by FDA concern over side-effects that include anxiety and depression.

While Acomplia and similar drugs under development turn CB1 receptors off to produce weight-loss, Dr. Raymond DuBois of M.D. Anderson said "turning CB1 back on and then treating with a cannabinoid agonist could provide a new approach to colorectal cancer treatment or prevention."

"Potential application of cannabinoids as anti-tumor drugs is an exciting prospect, because cannabinoid agonists are being evaluated now to treat the side-effects of chemotherapy and radiation therapy," DuBois said.

But the researchers also treated the mice with rimonabant, and said "mice with CB1 blocked in this manner also showed an increase in the number and size of polyps."

The researchers said a patient's risk for colorectal cancer should be assessed when use of Acomplia is being considered.