A Healthesystems publication

Spring 2012

Medical Marijuana and Drug Interactions in Workers’ Comp: Potentially Problematic

As “medical marijuana” or cannabis gains more mainstream traction and publicity about being used for medical purposes, it also is becoming an issue for workers’ compensation payers. Clinically, the most important issue to address is which problems might arise if medical marijuana is combined with the most commonly used drugs in workers’ comp?

The National Institute of Drug Abuse reports that over 16.7 million Americans reported using marijuana at least once in the prior month, making the studies of the potential effects, adverse effects, and interactions with marijuana a concern.1 Most of the recently reported data has focused on potential medicinal benefits, while little has been published regarding the potential interactions and adverse events associated with the use of marijuana and other related medications.

Studies have been conducted with Marinol and Cesamet® (nabilone), the two FDA-approved orally administered synthetic cannabinoids, and moderate interactions with opioids were noted. Orally ingested marijuana interacts with many drugs commonly used in the workers’ comp population by interfering with drug metabolism. Smoked marijuana induces an additional enzyme frequently involved with drug metabolism, raising additional concern with drug interactions compared to orally taken synthetic cannabinoids.2M Current studies that discuss oral cannabinoids do not necessarily apply to the inhaled forms of cannabis, adding an additional layer of unknown reactions.

Evidence shows that opioid and cannabinoid receptors may enhance or inhibit one another, though data has not been consistent across species. In rodents and monkeys, opioid consumption such as morphine increased desire for cannabis use. One study found that use of naltrexone, an agent that blocks the effects of opioids in humans, increased the patient’s perceived level of marijuana intoxication.3,4 These patients also experienced increased cardiovascular effects. This demonstrates that marijuana does not work at all of the receptor sites where opioids work, and as such may cause unexpected adverse effects. Medications that contain naltrexone include Embeda® (morphine-naltrexone) and ReVia® (naltrexone).

Both marijuana and opioid medications are CNS depressants and may have additive effects and cause over-sedation. Caution should be used when combining CNS depressants with marijuana, as this may worsen the adverse effects from opioids. While it would seem that the over sedation seen with opioid and marijuana use would be related to increased blood levels of opioids, a recent study did not support this theory. Results reported significantly decreased opioid maximum concentrations and a delay in time to reach maximum concentration levels.5 This information further reinforces the idea that interactions with marijuana are unpredictable.

In addition to the increased CNS adverse effects with opioids, other drug interactions have been reported. While some are simply case based, consideration of potential interactions is warranted and ongoing research needed. Other examples of interactions include:

There have also been reports of adverse reactions such as toxicity when drugs that are metabolized via the same pathway are used in conjunction with cannabis. Examples of these medications include tricyclic antidepressants and selective serotonin reuptake inhibitors (SSRIs). Other adverse CNS effects exist with the use of benzodiazepines, alcohol, barbiturates and antihistamines.6

Data from the National Survey on Drug Use and Health (NSDUH) indicates an association between cannabis use and nonmedical use of opioids. Likewise, an association was found between use of cannabis and medical use of opioids (data was limited to states without medical marijuana laws).7 This implies a greater risk of drug misuse when cannabis and opioids used together. Prescribers should be cautious when combining medications and look for signs of abuse, addiction and diversion.

Patient interviews and drug screens are potential ways to assess marijuana use. This information is important when providing patient care to identify both potential addictive behaviors, as well as medication interactions. Marijuana interactions should be evaluated with all concurrent prescription medications.

Marijuana interactions should be evaluated with all concurrent prescription medications.

Clearly, there are concerns and possible contraindications for medical marijuana use in the workers’ compensation population. Though a number of states have removed state-level criminal charges for marijuana possession and use by patients with certain medical conditions, the FDA has deemed marijuana as a Category I agent due to its high abuse potential. There is currently no accepted medical use in therapy in the United States, and there is a lack of accepted safety data.8

Healthesystems is closely tracking this issue, ensuring clients are aware of the potential for medical marijuana’s impact on other prescription drugs. We believe that working with clients to find ways to limit this type of risk is a critical part of the workers’ compensation care continuum.



1 -National Institute on Drug Abuse[homepage on the internet]. Bethesda, MD. Available at http://www.nida.nih.gov/DrugPages/Marijuana.html. Accessed February 22, 2012.
2 -Reisfield G. Medical Cannabis and Chronic Opioid Therapy. J Pain Palliat Care Pharmacother. 2010;24:356–361.
3 -Haney M, Bisanga A, Foltin RW. Interaction between naltrexone and oral THC in heavy marijuana use. Psychopharmacology. 2003;166:77-85.
4 -Cooper ZD, Haney M. Opioid antagonism enhances marijuana’s effects in heavy marijuana smokers. Psychopharmacology (Berl). 2010 Aug;211(2):141-8.
5 -Abrams DI, Shade SB, Kelly ME et al. Cannabinoid-Opioid interaction in chronic pain. Clin Pharmacol Ther 2011;90:844-851.
6 -Lindsey WT, Stewart D, Childress D. Drug interactions between common illicit drugs and prescription therapies. Am J Drug Alcohol Abuse. 2012. Available at: http://informahealthcare.com/doi/pdf/10.3109/00952990.2011.643997. Accessed February 22, 2012.
7 -Reisfield G. Medical Cannabis and Chronic Opioid Therapy. J Pain Palliat Care Pharmacother. 2010;24:356–361.
8 -Clinical Pharmacology [database online]. Tampa, FL: Gold Standard, Inc; 2011. Available at: http://www.clinicalpharmacology.com. Accessed January 18, 2011.
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