More people in the U.S. are buying and using weed since more states began legalizing marijuana, while the kind of weed they can purchase has become even stronger.

Scientists who study marijuana and its effects on the body now have concerns, in addition to emergency room doctors who say they’re starting to see more patients coming into the ER with marijuana-associated issues.

According to the National Survey on Drug Use and Health, 26 million Americans ages 12 and older reported that they were current marijuana users in 2017. Though it’s not clear how many users have experienced serious health issues from highly potent weed and many potential risks are still unknown, scientists are beginning to learn more about a few of them.

The potency of marijuana depends on the amount of delta-9-tetrahydrocannabinol, or THC, which is the main compound that is responsible for the drug’s psychoactive effects.

The U.S. Drug Enforcement Administration seized pot products and further did a study which found that the potency had increased from nearly four percent THC in 1995 to roughly 12 percent in 2014. By 2017, another study discovered the potency of illicit drug samples increased to 17.1 percent THC.

“That’s an increase of more than 300% from 1995 to about 2017,” says Staci Gruber, director of the Marijuana Investigations for Neuroscientific Discovery (MIND) program at the Harvard-affiliated McLean Hospital in Belmont, Mass. “I would say that’s a considerable increase.”

Products with concentrated forms of cannabis, such as hash and hash oil, can have as much as 80% to 90% THC, she adds. “I think most people are aware of the phenomenon that ‘this is not your grand daddy’s weed,’ Gruber says. “I hear this all the time.”

Some people might not be aware of the potential health risks of highly potent weed. Gruber said that the negative effects of cannabis have primarily been isolated and localized to THC, so higher levels of THC may, in fact, have a greater risk for a more negative outcome.

“In general, people think, ‘Oh, I don’t have to worry about marijuana. It’s a safe drug,’ ” says Nora Volkow, director of the National Institute on Drug Abuse. “The notion that it is a completely safe drug is incorrect when you start to address the consequences of this very high content of 9THC.”

“When someone takes marijuana at a low [THC] content to relax and to stone out, actually, it decreases your anxiety,” she says. But high concentrations can cause panic attacks, and if someone consumes high-enough levels of THC, “you become full-blown psychotic and paranoid.”

Marijuana can also have a similar paradoxical effect on the vascular system. “If you take low-content THC it will increase your blood flow, but high content [THC] can produce massive vasoconstriction, it decreases the flow through the vessels,”  Volkow said.

Low concentrations of THC can be used to treat nausea symptoms in cancer patients who are undergoing chemotherapy. However, Volkow says that “patients that consume high content THC chronically came to the emergency department with a syndrome where they couldn’t stop vomiting and with intense abdominal pain.” This condition is called cannabinoid hyperemesis syndrome.

“The typical patient uses [inhales] about 10 times per day … and they come in with really difficult to treat nausea and vomiting,” says 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.”

While the number of people who’ve had the syndrome is small and scientists are unsure what amount of THC can trigger it, Monte says he and his colleagues documented an increase of cases at emergency rooms in Colorado since marijuana was legalized there five years ago. Monte and his team also found that cyclical vomiting cases made up about 18% of inhaled cannabis-related cases at his ER.

Monte says 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 patients show up at the ER due to “intoxication” from too much marijuana, while the majority of these cases are from inhaling cannabis, though consuming edibles are mainly associated with more psychiatric visits.

“We’re seeing an increase in psychosis and hallucinations, as well as anxiety and even depression and suicidality,” Monte says.

“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.”

He added that not everyone is at an equal risk. “Many many people use cannabis safely,” he says. “The vast majority don’t end up in our emergency department.”

Patients should educate themselves whether using recreationally or medically and be cautious while using. Avoiding higher THC products and using infrequently can also help reduce risk, Volkow adds. “Anyone who has had a bad experience, whether it’s psychological or biological, they should stay away from this drug,” she notes.

“You have to know what’s in your weed,” Gruber says. “Whether or not it’s conventional flower that you’re smoking or vaping, an edible or tincture, it’s very important to know what’s in it.”