How Marijuana Affects Prescription Drugs

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Marijuana is legal in many states now, for medicinal or recreational use and health care providers and users should be aware of potential drug interactions.

Enzymes in the body metabolize (break down) drugs and make them inactive. That’s why drugs wear off after a few hours or a day or so. When another medication interferes with an enzyme that metabolizes a drug, it alters the way the body inactivates the drug. This can either cause the drug to be less effective, or it can magnify the effects, depending on the interaction.

Marijuana contains two cannabinoid compounds that are therapeutically significant, CBD (cannabidiol) and THC (tetrahydrocannabinol).

THC causes the body to produce more of an enzyme called CYP1A2. So THC can decrease the effects of drugs like clozapine, duloxetine, cyclobenzaprine, olanzapine and haloperidol which are metabolized by this enzyme.

CBD inhibits the enzymes CYP3A4 and CYP2D6 so it can increase the effects of macrolide antibiotics (erythromycin, clarithromycin), benzodiazepines (alprazolam, diazepam), cyclosporine, calcium channel blockers (verapamil, amlodipine), and sildenafil. CBD can also increase the effects of antidepressants (fluoxetine, sertraline), beta-blockers (propranolol, atenolol) and opioids (codeine, oxycodone).

THC and CBD can increase levels of warfarin and increase the INR.

Smoking cannabis can decrease theophylline levels.

Cannabis has additive sedative effects when used with alcohol, benzodiazepines, or barbiturates.

Some drugs may increase the effects of THC and CBD, including clarithromycin, antifungals (ketoconazole, itraconazole), antivirals (ritonavor, saquinavir), verapamil, and grapefruit juice.

Frequency of marijuana use varies among different users, but smoking more than 2 joints per week is enough to cause drug interactions.

For more information about drug interactions, click here and here

For information about marijuana click here

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