Cannabis is generally considered to be safe to use for most people. But there are very rare cases where pot can make people sick. The most well-known weed-related illness is probably cannabis allergies, but itchy noses and red eyes don’t even hold a candle to cannabinoid hyperemesis syndrome (CHS).

Cannabis hyperemesis is an incredibly rare and surprisingly controversial disease that affects a very small portion of cannabis users. It’s associated with heavy marijuana use and leads to bouts of excessive vomiting and persistent nausea.

What might be confusing about this disease is that cannabis is often regarded as a treatment for nausea—not a cause. One of the drug’s medical benefits that has been most touted by medical cannabis advocates for the last three decades is its ability to soothe nausea in cancer patients undergoing chemotherapy. In 2021, researchers at the University of New Mexico found that 96 percent of medical cannabis patients who turned to the drug to treat symptoms of nausea reported relief within an hour of consuming marijuana.

Because of this facet of cannabis, the idea that it could cause some people to become nauseated seems counterintuitive. There could be people suffering from CHS that are completely unaware of it. It doesn’t help that CHS can suddenly activate without warning in adults who have regularly used cannabis for years. In fact, symptoms for CHS appear only in those who have used cannabis heavily for many years and will disappear completely between episodes.

The illness is believed to present itself in three stages:

During the prodromal phase, victims experience symptoms like early morning nausea and abdominal pain. Most people follow normal eating patterns during this phase, but some will avoid food.

This period is followed by the hyperemetic phase, in which victims will suffer from continuing nausea, repeated vomiting episodes and severe weight loss. During this phase, many people will stop eating as much food and may suffer dehydration from fluid loss.

The third period is the recovery phase. Eating habits return to normal during this time and symptoms disappear. This phase generally lasts until the person uses cannabis again.

Treating the disease is simple enough. Patients are given anti-nausea drugs and after halting all marijuana consumption, they will usually return to normal within days. Frequent hot showers also seem to help mitigate discomfort associated with CHS, and compulsive showering is even considered a symptom of the illness.

The good news is that CHS will probably never become a concern for most cannabis users and you likely have nothing to worry about. But considering its knack for popping up unexpectedly, it’s probably a good idea for regular cannabis users to remember the signs.