John Vizcaino / Reuters Doctors are stumped by the science behind cannabinoid hyperemesis syndrome.

A mysterious marijuana-related illness is popping up with increasing frequency in hospital emergency rooms, particularly in states where cannabis is now legal.

The symptoms are severe abdominal pain and violent vomiting — and most doctors are initially stumped when they encounter patients with the problem.

The illness is cannabinoid hyperemesis syndrome, which is linked to heavy, long-term use of marijuana, according to experts. For some reason, the nausea and vomiting of CHS can be relieved with hot showers or baths, which can serve as an important hint for physicians trying to diagnose a patient.

The condition is especially puzzling considering cannabis’ accepted reputation as an anti-nausea agent; two cannabinoid-based medications are FDA-approved for alleviating nausea in people undergoing chemotherapy for cancer, while nausea is a common reason for doctors to prescribe cannabis in states that have legalized medical marijuana.

Since 2009, when the federal government relaxed its stance on medical marijuana, emergency room diagnoses for CHS in two of Colorado’s hospitals nearly doubled, according to a study co-authored by Dr. Kennon Heard, a physician at the University of Colorado Hospital in Aurora. Now that cannabis is also legal for recreational use in the state, “we are seeing it quite frequently,” Heard told CBS News. “My colleagues are seeing this on a daily to weekly basis.”

Emergency rooms in other areas where cannabis is legal are also reporting more cases of CHS. Georgetown University Hospital in Washington, D.C., and Harborview Medical Center and the University of Washington Medical Center in Seattle are among hospitals reporting an uptick.

CHS was first described in 2004 by scientists in Australia who published a small study about a group of 19 patients complaining of nausea, vomiting and chills. These participants were heavy long-term marijuana users (the study described the behavior as “chronic cannabis abuse”) who had developed a cyclical vomiting illness after several years of cannabis use. The study lost 10 patients to lack of consent or confounding factors, but the scientists found that seven of the patients who stopped using cannabis were able to stop the vomiting, three did not abstain and continued to have vomiting episodes, and three abstained for a short period and began vomiting again when they resumed using cannabis.

The researchers also noticed that nine of the patients were compulsive bathers who took an abnormal amount of hot showers or baths, even waking up in the night to do it. Their compulsion was not due to psychosis, the researchers wrote, but because the patients had discovered on their own that their nausea, vomiting and abdominal pain would subside just a few minutes after submersion in the hot water.

Since that study was published, scientists have not been able to pinpoint the exact cause of the syndrome, and only know that abstaining from cannabis generally helps people stop vomiting.

“The science behind it is not clear,” Heard told the Denver Channel. “The most likely cause is that people using marijuana frequently and in high doses have changes in the receptors in their body, and those receptors become dysregulated in some way, and it starts causing pain.”

Dr. David Steinbruner, an emergency room physician at Memorial Hospital in Colorado Springs, believes it’s likely triggered with a significant amount of marijuana. “The corollary would be alcohol. So small amounts may be fine for people, but over a long time it will cause all kinds of problems,” Steinbruner told KDRO-TV.

In its most severe form, the illness can lead to kidney failure — but symptoms stop within days of ending marijuana use. However, there is no research on how common CHS is among people who use cannabis, what the risk factors are for developing it and how often marijuana for medical use leads to excessive cannabis use. Heard, who published the 2015 study showing the number of CHS cases rise after Colorado legalized marijuana, noted in his report that despite the high use of pot in the state, the absolute number of cases still remains very low, “underscoring that CHS is a relatively uncommon condition.”

“Patients are given IV fluids and medication to resolve the vomiting and help with the pain,” explained Heard. “But the treatment is really to stop using marijuana, or at least to cut back severely, and that’s really the only way to make it better.”