Just like ragweed and birch trees, marijuana plants may trigger allergic reactions in some people, according to a new review of previous studies.
And because of the increasing use and cultivation of marijuana that has followed in the wake of legalization in some places, allergies to marijuana may be on the rise, experts say. People can be allergic to the plant's pollen, or to its smoke.
"Although still relatively uncommon, allergic disease associated with [marijuana] exposure and use has been reported with increased frequency," wrote the authors of the review, published March 3 in the journal Annals of Allergy, Asthma & Immunology.
In fact, allergies to marijuana have likely gone underreported, because of marijuana's illegal status, said Dr. Purvi Parikh, an immunologist of the Allergy & Asthma Network, a nonprofit organization that promotes allergy research and education.
"Now as the prevalence [of marijuana use] is increasing, and with the legalization in many states, it is going to become increasingly more common, and all these cases will surface that were not recognized before," Parikh said. [9 Weirdest Allergies]
People who are allergic to the marijuana plant's pollen or smoke may get symptoms such as a runny nose, inflammation of the nasal passages, and coughing and sneezing, according to the review. Some people who have touched marijuana have developed hives, and itching and swelling around the eyes. There have also been reports of asthma triggered by exposure to its pollen, according to the review.
One patient who ate hemp seed-encrusted seafood experienced the severe allergic reaction called anaphylaxis, which affects the whole body and is potentially fatal, according to the review. (The man's doctors later excluded the seafood as the cause of his allergic reaction through testing.) In another case mentioned in the review, a person who used marijuana intravenously also experienced anaphylaxis.
Some people have experienced allergic reactions while handling marijuana at work, according to the review. A bird breeder developed allergy symptoms after feeding hemp seeds to his birds, and one unlucky medical marijuana grower, who previously was able to smoke pot recreationally, suddenly developed hives and itching after handling the plant.
For some marijuana users, it is not only the plant itself that may cause an allergic reaction. Pot can become very moldy when it is being stored, and people who are allergic to mold may have reactions, Parikh said.
Some people could even experience reactions to both the plant and mold, as many people with allergies are allergic to multiple substances, she said.
The review pointed out two studies conducted decades apart in Omaha, Nebraska, where the Cannabis plant grows widely and is cultivated commercially. In the studies, researchers looked at how common cannabis allergies were among people in the area. In the first study, published in 1940 in the Nebraska Medical Journal, researchers found that 22 percent of 119 patients with allergy symptoms were allergic to hemp pollen.
In the later study, published in 2000 in the journal Annals of Allergy, Asthma & Immunology, investigators found that 61 percent of 127 patients with allergies in Omaha were allergic to hemp, according to the report.
People who live in areas where large quantities of marijuana plants are grown may be particularly prone to experiencing allergic reactions to the pollen, Parikh said. "The quantity makes a big difference in the prevalence of allergies."
Follow Agata Blaszczak-Boxe on Twitter. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.
