More than 20 states have OK’d marijuana in one form or another. As pot has risen “above board” in a wider legal and cultural sense, some issues that no one every talked about have come to light.

One of them: Weed allergies. “Recently we’ve seen several cases in our practice,” says William S. Silvers, MD, a clinical professor of medicine at the University of Colorado and an allergist in private practice.

Silvers recently published a short piece on marijuana allergies in Annals of Allergy, Asthma and Immunology. He says it’s likely weed allergies are underreported because many people still feel uncomfortable talking with their docs about a pot habit.

What does a marijuana allergy look like? Silvers lists symptoms like sneezing, coughing, a runny nose, itchy or red eyes, swollen eyelids, and asthma or trouble breathing.

You could joke that everyone coughs or has red eyes after smoking weed. But for people with an allergy, these reactions can grow worse the more they use. “It may take five or 10 or more exposures, but once you have a reaction it probably isn’t going away,” Silvers says.

Marijuana is a weed and a pollen, he explains. As a result, people with plant or pollen allergies are most likely to experience a reaction—especially if they’re smoking weed. At the same time, Silvers says people who have no issues inhaling pot may have an anaphylactic reaction from swallowing edibles. That could lead to skin rashes, vomiting, or serious problems breathing.

What should you do about it? If smoking weed gives you a runny nose or itchy eyes, over-the-counter antihistamines (Benadryl, Zyrtec, Claritin, to name some common brands) can help. Switching to “ingestibles” or a vaporizer may also solve your problem, though Silvers says that’s really just his best guess. (Allergies aside, he’s quick to add that vaping is not necessarily any better for you than smoking.)

If you think your weed habit may be triggering asthma or trouble breathing, you really need to cut it out—or at least see a doctor, he says. “If you ignore an allergy, there’s the potential for it become more serious.”

As legalization leads to purer and more potent pot, Silvers says he expects to see more patients who have had reactions. Along with the recreational perks, “there are lots of good, valid medical reasons to use marijuana,” he says. The flip side of that is there may be some valid medical reasons for some people to steer clear of pot.

“The takeaway is some people will have to take a hard look at what they’re doing, and whether it’s safe for them to continue,” he says.

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