Feb. 21, 2017 — A small but growing number of heavy marijuana users around the world are getting a rare illness that causes vomiting and abdominal pain, and experts are at a loss to explain why.

Cases of cannabinoid hyperemesis syndrome (CHS), also known as cyclical vomiting syndrome, are rare, compared with the overall number of marijuana users, but incidents are on the rise in states that have legal marijuana like Colorado, where what is sold in stores and medical dispensaries is often much stronger than what was long available on the black market.

"When the decriminalization of medical marijuana occurred and the number of medical (marijuana) licenses in the state went up dramatically, we started seeing more and more cases of young, otherwise healthy people who would have multiple recurrent episodes of severe abdominal pain and vomiting and often end up in the emergency room," said Kennon Heard, MD, chief of medical toxicology for the University of Colorado Anschutz Medical Campus in Aurora.

Heard co-authored a study that found such cases doubled at two Colorado hospitals between 2009 and 2011, when the state saw a massive boom in its medical marijuana industry. He does not have updated statistics on cases since Colorado voters legalized recreational marijuana in 2012, but, he said, "If you talk to anyone who practices emergency medicine in the state of Colorado, they're going to tell you they've seen this and they see it fairly frequently, and we see it in our emergency department on a daily or weekly basis."

The problem is not limited to Colorado. Marijuana is now legal for recreational use in eight states and medical use in 28 states, and doctors from Arizona to Oregon to Canada have seen an increase in cases of the syndrome.

It was first discovered in 2004 in Australia, where drug laws are liberal and physicians saw more heavy users of marijuana having bouts of vomiting and abdominal pain that repeated every few weeks or months and lasted for years. All had been daily marijuana smokers for multiple years, according to a study. Quitting made the symptoms go away. And for some reason, the patients found relief from the symptoms with a hot shower or bath.