Prescription drugs are supposed to soothe our symptoms, cure us when we're ill and prolong our lives. But do some of our most popular products also increase our risk of cancer?
This is a question the Food and Drug Administration has a hard time answering. The uncertainty creates confusion and anxiety for both doctors and patients.
Controversy surrounds many of the most successful medicines on drugstore shelves. Although estrogen has been prescribed for over 50 years to relieve hot flashes and other menopausal symptoms, there are still unresolved issues about the drug's potential to increase the risk of breast cancer.
Many consumers were alarmed when a Canadian scientist, Dr. Lorne Brandes, announced in the July 1992 issue of Cancer Research that the anti-depressant Prozac (fluoxetine) promoted tumor growth in laboratory animals. In May 1994 he published similar research in the Journal of the National Cancer Institute concerning such antihistamines as Claritin (loratadine) and Hismanal (astemizole). The FDA didn't know what to make of these findings but the agency was sufficiently concerned to initiate additional research.
Part of the problem is that cancer is a complex set of diseases that can take a long time to develop. Experts don't yet agree about the best ways to test drugs and chemicals for their cancer-causing potential. Animal studies are widely used but questions remain about interpreting results. Even studies designed to determine the risk for humans don't always produce firm conclusions.
In the doctor's bible of prescribing information, the Physicians' Desk Reference, many drugs have scary warnings.
Metronidazole (Flagyl) is an extremely effective treatment for amebic dysentery and certain vaginal infections. It is also becoming popular in combined antibiotic therapy to cure stomach ulcers. But you will find a boxed warning: "Metronidazole has been shown to be carcinogenic in mice and rats . . . Unnecessary use of the drug should be avoided . . ."
We wonder what the FDA intends by this admonition since we cannot imagine physicians prescribing drugs unless they are necessary. People have worried whether this caution has any meaning for them. Although metronidazole has been linked to cancer in animals, there is still no way to tell if humans run a risk from short-term use.
Another controversial compound is lindane (Kwell). This insecticide has been banned in some countries and is carefully regulated in the United States. It is also sold as a prescription lice shampoo. Although some animal tests show this chemical may promote tumor formation, it's not clear if there is a danger for children. One public-interest group recently petitioned the FDA to ban lindane from lice shampoos.
Calls for removing drugs that cause cancer in animals may be premature. But without further studies and firmer results, physicians and patients are left in limbo. The FDA needs either to remove the warnings and reassure us that there is no risk, or determine how hazardous such drugs really are.
Q: My dermatologist prescribed spironolactone for excess hair growth on my face. When I looked it up, I was alarmed to read that it is a tumorigen in animals. Doesn't that mean it causes cancer? I don't know if it's worth the risk.
A: We have no way to tell if this diuretic poses a danger. We know of no long-term studies in humans that show if it increases the risk of cancer.
Q: I'm wondering about an interaction between the anti-coagulant properties of aspirin and garlic. I take half an aspirin daily to thin my blood. I understand garlic also helps prevent blood clots. Would adding garlic pills pose a danger of bleeding?
A: Like aspirin, garlic does seem to make the sticky parts of blood (platelets) less likely to clump together. Although there are no studies that show hemorrhage is more likely in people taking garlic and aspirin, some experts warn that there may be a potential for interaction.
Joe Graedon is a pharmacologist. Dr. Teresa Graedon is a medical anthropologist and nutrition expert.