The study, published in the Archives of Internal Medicine, suggests men and women who take such drugs are equally at risk.
Some smaller studies had previously shown a tentative link between painkillers and kidney cancer. But study researcher Eunyoung Cho, ScD, assistant professor of medicine at Harvard Medical School, says this is the largest study to look at the relationship between the disease and the class of drugs called nonsteroidal anti-inflammatory drugs (NSAIDs).
Cho says that the overall risk is still quite low. She does not think that people should stop taking NSAIDs in the short term or that doctors should stop recommending them to patients who otherwise benefit from them.
Kidney cancer specialist Christopher W. Ryan, MD, who was not involved in the study, agrees.
"Occasional use for aches and pains is nothing to worry about," says Ryan, of Oregon Health and Science University's Knight Cancer Institute in Portland. "But one's risk of getting kidney cancer, even for a long-time user, is not high at all."
Measuring Risk of Long-Term Use
Cho and her colleagues analyzed the records of more than 77,000 women and nearly 50,000 men, all of whom were participants in one of two very large studies: the Nurses' Health Study and the Health Professionals Follow-up Study. The studies tracked the use of these medications for 16 years and 20 years, respectively.
Non-aspirin NSAIDs were used regularly at least two times per week by 19% of women and 6% of men.
The risk for both men and women who regularly took non-aspirin NSAIDs increased by 51% during the years analyzed. But a person's overall risk of getting kidney cancer is still small, as Ryan said. Of all the people in the study, 333 were diagnosed with kidney cancer. Still, the risk varied according to how long the medications were taken.
Those who took non-aspirin NSAIDs regularly for four to nine years had a 36% higher risk of kidney cancer compared to non-regular users. However, Cho says that that finding may have been due to chance. The only group at notable risk was made up of people taking them for more than 10 years. Their risk increased by nearly 300%.
"Not a large amount of people take them for that long," says Cho. "Short-term use is much more common."
Drug Industry Reaction
The Consumer Healthcare Products Association (CHPA), an industry group that represents over-the-counter drug manufacturers, says this class of drugs is both safe and effective when taken according to the label.
"The results of this study reinforce our guidance to consumers that long-term use beyond labeled directions should only be undertaken after consultation with a doctor, who can appropriately evaluate benefits/risks and monitor therapy closely," the CHPA says in a statement provided to WebMD.
For women, the more often they took them during a given month, the higher the risk. There was an 8% increase among women who took them up to four times a month. The increased risk to women who took them more than 15 days per month went up 86%. Dosage data was not available for male participants.
The American Cancer Society estimates that 60,920 new cases of kidney cancer will occur this year and will kill more than 13,000 people. Renal cell cancer, the type that Cho and her team studied, accounts for about 85% of all kidney cancers, which are often treatable if caught early.
While cancer of the kidneys is twice as common in men as it is in women, Cho's study found that both men and women were almost equally likely to develop the disease as a result of taking non-aspirin NSAIDs.
"Because we observed the association in two independent populations, it is unlikely to be a chance finding," the researchers write.
This study points out that previous research has linked non-aspirin NSAIDs with reduced risks of breast, prostate, and colorectal cancers, while others have reported NSAIDs in general to be associated with both acute and chronic kidney problems.
Cho says her study's results, while further establishing the link between kidney cancer and non-aspirin NSAIDs, need to be confirmed by additional research and they should not cause anyone to panic.
"The risk is something to keep in mind," she says, "but there need not be a change in practice at this point."