Common Drugs and Marijuana Don’t Always Mix


In these stressful times, many people are looking for ways to relieve their anxiety and relax. For some, using marijuana might offer some respite. However, marijuana—or more specifically, a compound in the plant that produces its intoxicating effects called tetrahydrocannabinol (THC)—can have adverse effects when combined with some prescription drugs and vice versa, warns a new report from the University of Toronto. If you use marijuana, for either recreational or medical purposes, be aware of these possible drug interactions: 

  • Some drugs can increase THC levels. The combination of marijuana with drugs used to inhibit liver enzymes can increase sedation and other effects of marijuana, as well as increase the risk of liver damage. These medications include verapamil and amiodarone (used to treat high blood pressure and irregular heartbeat, respectively), certain antibiotics, ketoconazole (an antifungal), and fluoxetine (an antidepressant).
  • Marijuana can increase levels of some drugs. Marijuana can interfere with the metabolism of some drugs by suppressing some liver enzymes. Clobazam (an antiseizure drug) and tacrolimus (an immunosuppressant) levels, for example, increase several-fold when combined with THC. An anticoagulant like warfarin might reach dangerously high levels in the body, which can lead to bleeding.
  • Smoking marijuana can decrease levels of other drugs from the body at a faster rate than normal, making them less effective or even ineffective. Examples of such drugs include theophylline (a bronchodilator) and olanzapine (an antipsychotic).
  • Additive effects can occur with other drugs. Marijuana can intensify the effects of some drugs, including sympathomimetics (used to treat low blood pressure), alcohol and opioids, and anticholinergics such as antihistamines and antiparkinson agents. A rapid heart rate, high blood pressure, drowsiness, loss of coordination, slurred speech, and confusion are common consequences.
  • There are other potential “red-flag” interactions. More research is needed, but other drugs that might interact with marijuana include nervous system depressants (including sleep aids, antianxiety drugs, and other sedatives), and clozapine (an antipsychotic).

 WHAT ELSE YOU SHOULD KNOW: The U.S. National Survey on Drug Use and Health reports that 4.2 percent of adults ages 65 and older used marijuana in 2018, up from 0.4 percent in 2007. The study findings were reported online March 2 in the Canadian Medical Association Journal (CMAJ).

WHAT ELSE YOU SHOULD DO: Be honest with your doctor about any recreational marijuana use. The list of medications above is not inclusive, so you should review the drugs you take with your doctor to look for any potential interactions.