TUESDAY, March 3, 2020 (HealthDay News) -- People who use marijuana either medicinally or recreationally should remember that pot can have potentially harmful interactions with medications they take regularly, a new report warns.

This is particularly true of older folks, who often are on many different drugs to manage a variety of chronic ailments, said lead researcher Tony Antoniou. He's an associate professor in the department of family & community medicine at the University of Toronto.

"There are some reports showing that older patients are one of the fastest-growing groups of people who are taking cannabinoids," Antoniou said. "Because they typically take more medications for a variety of illnesses, we thought it would be important to raise awareness about the potential for interactions."

The proportion of seniors aged 65 and older who use marijuana increased 75% in recent years, rising from 2.4% to 4.2% between 2015 and 2018, according to data from the U.S. National Survey on Drug Use and Health.

Doctors and pharmacists are mainly worried about how marijuana might alter the way that prescription medications are metabolized by the liver, Antoniou and his colleagues said.

"It's still a drug, and drugs whether they're naturally derived or produced in a lab can interact with one another," Antoniou said.

Marijuana can either speed up or slow down how quickly the liver processes other drugs, Antoniou said.

As a result, these other medications might be broken down too quickly to be effective, or so slowly that they build up to dangerous levels in the body.

One troubling example is the blood thinner warfarin, which typically is prescribed to prevent blood clots that can lead to heart attacks or strokes.

"There are some cases of people who were previously stabilized on it who started using marijuana and suddenly they developed side effects like bleeding," Antoniou said.

The prescribed dosage of drugs like warfarin have to be very specifically dialed in for individual patients, and marijuana use can interfere with that, said Barbara Young, editor of consumer medication information for the American Society of Health-System Pharmacists.

"There's a very narrow range you want to hit so you're preventing the clotting but you're not increasing the risk of bleeding," Young said. "Any changes to that would be of great concern."

Asthma medications are another concern, Antoniou added. Studies have shown that breakdown of some asthma drugs occurs more rapidly in people who regularly smoke marijuana, reducing the medications' effectiveness.

The interaction can run in the other direction as well, with prescription medications amplifying the effects of pot.

For example, the antifungal drug ketoconazole has been shown to nearly double the levels of marijuana chemical compounds in the body, researchers said. Similar effects might be seen with other commonly used drugs, like antibiotics.

Marijuana also can add to the impact of drugs that have a sedative effect, such as over-the-counter antihistamines, antidepressants and sleeping medications, Young said. People might end up a lot more lethargic and clumsy than they would have otherwise.

Antoniou and Young said the best way people can stay safe is to be honest with their doctor and pharmacist about their marijuana use.

"It would be good if people who are taking marijuana or trying it to sit down with their doctor or pharmacists and review their medications and see if there are any potential interactions that could be flagged," Antoniou said. "I would encourage that kind of conversation to be happening, especially for people who are taking many different types of medication."

One hurdle is that there's not been a lot of research on marijuana interactions with other medications, Antoniou added.

"There's not a lot of good data to guide us, so a lot of it will be based on understanding the pharmacology of marijuana, THC [the chemical in marijuana that provides a high], CBD [cannabidiol], and the pharmacology of whatever other medications individuals are taking," Antoniou said.

The report was published online March 2 in the CMAJ (Canadian Medical Association Journal).

More information

The AARP has more about seniors and medical marijuana.