More than 26 million Americans age 12 and older used marijuana in 2017, and more and more of them are looking for that extra high only possible with higher concentrations of THC (tetrahydrocannabinol), the main psychoactive compound in weed.

This growing yen for stronger weed carries with it the seeds of becoming a serious national problem, said the National Survey on Drug Use and Health. What’s clear is that levels of THC in weed and weed products have risen dramatically in the U.S. from 1995 to 2017 — and they could soar even higher.

One study of illicit weed products found their potency increased from about 4 percent THC in 1995 to about 12 percent in 2014. By 2017, the potency of illicit drug samples had gone up to 17.1 percent THC.

"That's an increase of more than 300 [percent] from 1995 to about 2017," according to Staci Gruber, director of the Marijuana Investigations for Neuroscientific Discovery (MIND) program at the Harvard-affiliated McLean Hospital in Massachusetts."I would say that's a considerable increase."

She said some products with concentrated forms of cannabis, like hash and hash oil, can have as much as 80 percent to 90 percent THC.

This spike seems to be driven by the fact that as more people become users, there’s a craving among some of them to try something new and stronger. Medical authorities say they don’t know how many users have had serious health issues arising from strong weed.

Experience with patients undergoing treatment from the effects of strong weed has revealed valuable insights, however. A particularly worrying problem is the rising number of pot users being admitted to emergency rooms (ERs) suffering from “cannabinoid hyperemesis syndrome.”

Nora Volkow, director of the National Institute on Drug Abuse, said patients that consume high content THC can’t stop vomiting and suffer from intense abdominal pain.

While the number of people who've had the syndrome is small, this number is rising and cannabinoid hyperemesis syndrome has also killed people.

"The typical patient uses (inhales) about 10 times per day ... and they come in with really difficult to treat nausea and vomiting," noted Andrew Monte, an associate professor of emergency medicine and medical toxicology at the University of Colorado's school of medicine. "Some people have died from this ... syndrome, so that is concerning."

Scientists don't know exactly how high levels of THC trigger the syndrome, but the only known treatment is stopping cannabis use. Some relief can be attained by taking a hot shower.

Monte and his colleagues have documented a rise in the number of cases at ERs in Colorado since marijuana was legalized here five years ago. A study by Monte and his team found cyclical vomiting cases comprised about 18 percent of inhaled cannabis-related cases in his ER.

They also found that throughout Colorado, the overall number of ER cases associated with cannabis use has increased. Monte said his ER has "seen an approximately a three-fold increase in emergency department visits just by frequency. It doesn't mean we're getting overwhelmed by these visits due to cannabis, it's just that means that there are more patients overall."

Most people it his ER are there because of "intoxication" from too much pot, either straight or mixed with other drugs. Monte said the bulk of these cases are caused by inhaled cannabis. On the other hand, edibles are associated with more psychiatric cases.

"We're seeing an increase in psychosis and hallucinations, as well as anxiety and even depression and suicidality," said Monte, who thinks the increased potency of marijuana plays a role in all these cases.

"Whenever you have a higher dose of one of these types of drugs, the patient is at a higher risk of having an adverse drug event. If the concentration is so much higher ... it's much easier to overshoot the low-level high that they're looking for."

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