Tuesday night, I had what my pharmacist describes as an allergic reaction to methotrexate. Within two hours of injecting it into my thigh, I developed nickel- and quarter-sized hives on my knee and elbow pits. My breath was short. My heart palpitated.

With any other patient, with any order condition, with any other drug, these symptoms would be a classic allergic reaction. Any doctor would tell you to stop taking it. When you’re dealing with autoimmune diseases, this could almost mean anything.

A classic allergic reaction is caused by the immune system’s hypersensitivity to a typically harmless substance. Treatment for such a reaction is usually a drug that mildly suppresses the immune system. For example, diphenhydramine (Benadryl) is an antihistamine that treats all sorts of mild allergy attacks and cold symptoms. It is a common ingredient in NyQuil and other over-the-counter cold medicines. Doctors use another common immune system suppressant, prednisone, to treat asthma and rheumatoid arthritis. I also take it to treat my dermatomyositis.

Methotrexate is a heavy-duty immune system suppressant. The irony of an immune system suppressant causing an allergic reaction was not lost on my pharmacist or my internist.

Such a reaction after being on the medication since October is odd, but according to my pharmacist, very possible. Also odd: shortness of breath is a symptom of untreated dermatomyositis. However, hives are not. And this is the second week I have had them after injecting myself with methotrexate.

Because I only take the drug once a week, I have already discontinued it. I made an appointment with my rheumatologist for next week. I will most likely switch to a different medication, most likely, another immune system suppressant, azathioprine.