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Co-administration of Cannabis Based Drugs: The Evidence

When creating a rational cannabis care plan for patients, a multitude of different factors need to be considered. One such factor to be carefully considered when trying to establish an appropriate dose for patients is co-administration with other drugs the patient is already taking prior to beginning cannabis-based treatment.

As cannabinoids, including CBD and THC, share breakdown enzymes with a number of other health supplements and pharmaceutical drugs, it is a possibility that treatment that occurs at the same time as use of another drug or herb may have an effect on the metabolism and absorption of the cannabis-based medicine.

Indeed, co-administration of cannabis-based medicines with other drugs or herbal remedies may result in a delayed metabolism with higher exogenous cannabinoid levels or alternately, a sped-up metabolism with reduced exogenous cannabinoid levels.

Although the effect cannabis has on drugs and herbal remedies is ‘unquantifiable’ in many cases, there have been a few notable published reports on specific drug and herbal interactions:


  • THC and CBD have been shown to increase warfarin levels (Yamaori et al 2012).
  • Frequent use of cannabis has been associated with increased INR, International Normalised Ratio – a blood test used to check how long blood takes to clot, as evidenced anecdotally and in one published case report in 2019.


  • Smoked cannabis can decrease theophylline levels (Stout and Cimino 2014).


  • Alcohol may increase THC levels as shown by Hartman in a study in 2015.

Indinavir or Nelfinavir

  • Smoked Cannabis had no effect on these drugs (Abrams et al 2003).

Docetaxel or Irinotacan

  • Cannabis infusion (tea) had no effect on these drugs (Engels et al 2007).


  • CBD was shown to increase clobazam levels in children treated with the substance for epilepsy (Geffrey et al 2015).

Immunotherapy agents for non-small cell lung cancer, renal cancer and melanoma

  • A retrospective observational study from Israel highlighted a decreased response to the drug but showed no difference in progression-free or overall survival.

St John’s Wort

  • There have been no published studies on specific interaction with CBMPs or THC/CBD, but CBMPs and St John’s Wort share multiple CYP enzyme pathways.
  • St John’s Wort is one of the most well-known supplements for inducing enzymes and altering the effects of drugs when co-administered with them, so interaction here may be likely.

Overall, creating a cannabis care plan for a patient new to cannabis-based medicinal products should be an extremely careful process that requires much caution from healthcare professionals as a great number of factors, including co-administration circumstances, need to be considered in addition to the patient’s condition and outright symptoms.

Further investigation and research into medicinal cannabis and alternative medical options is thoroughly encouraged by The Academy, particularly through the use of our own online coursesevidence base and whitepapers.

The rest of our resources are available on our website. We urge anyone considering use of medical cannabis products to consult with a trained medical professional prior to beginning use.

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