The blood pressure drug you’re taking to protect your health could actually be endangering it—by making you more likely to get a deadly form of skin cancer! Here’s the shocking story…

Hydrochlorothiazide (HCTZ), a diuretic often called a “water pill” because it makes your body excrete more fluid, is one of the most popular blood pressure drugs. It’s well known that the drug has a photosensitizing effect, meaning that it makes skin more sensitive to damage from the sun’s ultraviolet rays. In fact, a study from the University of Southern Denmark found that risk for nonmelanoma cancer of the lip was seven times higher in people taking HCTZ—and that 11% of the cases of this kind of lip cancer could be attributed to HCTZ use. On the other hand, there has been very little research into the implications for melanoma, the deadliest form of skin cancer, with HCTZ use.

In a new study, the researchers from the same university analyzed 10 years of data on about 19,000 people (ages 18 to 90) with melanoma and compared it with data on people who were cancer-free (although some had previously had nonmelanoma skin cancer). Besides checking for HCTZ use, the researchers also checked for use of several other types of blood pressure drugs.

Results: Compared with people who did not have melanoma, people with melanoma were more likely to have taken high cumulative doses of HCTZ. In fact, they were 22% more likely to have taken a cumulative dose of 50,000 mg or more. Of particular concern to the researchers was that people who had taken the highest cumulative doses of HCTZ (100,000 mg or more) were 126% more likely to have nodular melanoma, a form of the disease that isn’t clearly tied to high sun exposure. None of the other blood pressure drugs studied were found to be related to melanoma risk at any dose.

If 50,000 mg sounds like a lot, consider that people prescribed blood pressure drugs generally need to take them for the rest of their lives. While a typical starting dose of HCTZ is 12.5 mg/day, doses up to 50 mg/day are prescribed. Taking just the lowest dose of 12.5 mg daily would result in a cumulative dose of 50,000 mg in 10 years—not such a long time in the scheme of things.

Bottom line: If you need to take a blood pressure drug, discuss the association between HCTZ and melanoma with your doctor. You’ll need to consider that HCTZ is generally well tolerated, cheap and effective. However, if you have already had skin cancer or are at higher risk for it (because of family history, light-colored skin or spending a lot of time outdoors, for instance), an alternative drug might be more appropriate for you. In any case, do not stop taking HCTZ (or any other blood pressure medication) without first talking to your doctor. And if you do take HCTZ, in addition to being especially vigilant about sun protection, be sure to get screened regularly for skin cancer. The earlier melanoma is detected and treated, the better the outcome. You can get a free skin cancer screening through the American Academy of Dermatology.