In recent years, immunotherapy has taken off as one of the most exciting developments in cancer treatment. Checkpoint inhibitors such as pembrolizumab (Keytruda) and ipilimumab (Yervoy) are one type of immunotherapy. These drugs are designed to target specific proteins on immune cells (T cells) which encourages the body’s own immune system to attack the cancer.

Checkpoint inhibitors are currently approved to treat a number of different types of cancer, including melanoma, lung cancer, bladder cancer, and Hodgkin’s lymphoma, among others. These drugs are often very effective, but they may also cause side effects that range from the relatively minor (fatigue and nausea) to life-threatening (organ damage).

Now new research, published in the journal Arthritis and Rheumatology suggests that these drugs may also increase the risk of rheumatologic conditions like inflammatory arthritis.

The researchers identified nearly 1,300 patients who had been given a checkpoint inhibitor at the Mayo Clinic in Rochester, Minnesota between 2011 and 2018. They found that 43 of them had since been diagnosed with rheumatologic condition such as inflammatory arthritis or myopathy (muscle disease).

Whether or not checkpoint inhibitors actually caused rheumatologic disease in these patients isn’t clear. The study authors speculate that at least some of the patients might have had inflammatory arthritis all along and that the cancer treatment somehow “unmasked” it.

The authors also say there’s no obvious reason why certain patients using checkpoint inhibitors develop a rheumatologic disease like inflammatory arthritis when most others don’t, though it’s probably not just a coincidence. There may be some underlying mechanism by which these drugs are triggering rheumatic disease, though scientists don’t yet understand what it is.

If your doctor has suggested a checkpoint inhibitor to treat cancer, you should ask about all possible side effects including this one. In certain cancers, these drugs can have a remarkable impact on prolonging life or even help some patients achieve remission, so there’s likely a good reason this was recommended as part of your cancer therapy. As with all medical treatments, you’ll need to weigh the pros and cons of all your options (with your doctor’s assistance) before making the best choice for you.