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One of the most important questions patients ask when starting medical cannabis therapy: will treatment interfere with my other medications? Let's explore cannabis as an adjunct therapy and lend some safeguards and best practices to the conversation.

Presently, there is relatively little scientific information known about the safety of individual cannabinoid compounds as compared to other common classes of synthetic medications. While this is the case, known serious adverse reactions and side effects are still rare with medical cannabinoids. Awareness of potentially harmful interactions between medical cannabis and medications used for physical and mental health conditions will help to safeguard you prior to, and during, medical cannabis therapy.

Understanding that THC and CBD components of medical cannabis are known to interact with common medications may help to avoid harmful and potentially life-threatening interactions.

To understand how medications interact in the body we will overview a key enzyme group found primarily in the liver called Cytochrome P450. For example, common enzymes include ones labelled CYP3A4, CYP2D6, and CYP1A2 that metabolize more than half of the medications we consume; these and many more have been, and continue to be, extensively studied.

Some medications are metabolized and interact with others in such a way that it causes the same enzyme to inhibit or induce the metabolism of the other. In other words, one or more competing medications can cause increases or decreases in the relative amount of another medication or substance. This may then change the effect, efficacy and safety profile of one or more substances and medications involved.

THC is a known inducer of the CYP1A2 enzyme, which means that THC may decrease the blood levels of other commonly prescribed medications, namely psychiatric and pain medications leading to a decreased effect of that medication at the standard dose that you maybe taking. The risk is that, because of the decreased efficacy of those medications, a person may take higher doses and cause potentially harmful and life-threatening side effects due to taking more of these potent medications during times when not taking THC.

CBD is a known inhibitor of the CYP3A4 and CYP2D6 enzymes. These enzymes metabolize many commonly used medications; CYP3A4 in particular is known to metabolize approximately one-quarter of all medications, including certain antibiotics, antivirals (and HIV medications), antihistamines, blood pressure medications, and cholesterol medications. CYP2D6 notably is responsible for metabolizing many psychiatric medications such as Selective Serotonin Receptor Inhibitors (SSRIs) and tricyclic antidepressants, antipsychotics (risperidone and haloperidol), certain beta blockers and opioids (particularly codeine and oxycodone). What this means is that CBD may increase the relative blood levels of these medications when used together, potentially causing harmful and life-threatening side effects.

Key life-saving medications that are commonly used by adults, such as blood thinners, anti-virals, antipsychotics, cholesterol and blood pressure medications that are taken at stable doses, may be adversely affected by the use of THC and CBD without taking into account the possible changes in the efficacy of all medications involved. Consuming alcohol and smoking tobacco cigarettes while taking prescribed medications and medical cannabis poses a greater risk. Alcohol, for instance, may increase THC levels and further change the metabolism and effect of other medications.

It is important to note that there are relative and absolute contraindications for using medical cannabis when taking medications for certain indications. Medical cannabis is relatively contraindicated for those with immune, heart, liver and kidney diseases due to potential changes in the efficacy of medications used for those purposes. Medical cannabis is absolutely contraindicated for those people with an allergy to THC or CBD, acute psychosis or any other unstable psychiatric conditions and pregnant women due to the same reasons. It is advised that you speak with your healthcare provider about any concerns of drug-to-drug interactions between your current medications and medical cannabis.

Please see How do I talk to my doctor about medical cannabis? for more information.

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