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Published: 2 March 2017

Medicines and Illicit Drug Interactions

Prescriber Update 38(1): 10
March 2017

Key Messages

  • Interactions between prescription medicines and illicit drugs can lead to adverse outcomes.
  • The possibility of interactions with illicit drugs should be considered when prescribing, and when patients present with suspected adverse reactions.

Patients often fail to inform healthcare professionals about the dietary supplements, complementary medicines or illicit drugs they are taking. However, each of these categories includes substances that can interact with medicines, sometimes with serious, even fatal consequences.

The United Kingdom’s medicines regulator (Medicines and Healthcare products Regulatory Agency, [MHRA]) recently received a Coroner’s report regarding the death of a man from a subarachnoid haemorrhage after a suspected interaction between citalopram and cocaine1. The UK’s Pharmacovigilance Expert Advisory Group identified plausible mechanisms that could lead to a subarachnoid haemorrhage from this interaction, including hypertension related to cocaine and an increased bleeding risk with citalopram1.

Interactions between prescription medicines and illicit drugs may result in adverse outcomes, including toxicity or a reduction in therapeutic effect2. The potential interactions with any illicit drug should be considered when prescribing medicines, as well as in patients who present with suspected adverse reactions to medicines1.

To ensure appropriate prescribing, an adequate patient history should be taken, including the current and recent use of all medicines (including non-prescription, complementary and alternative medicines)3. Open and effective communication is essential and should be encouraged between patients and healthcare professionals4.

The New Zealand Formulary includes potential interactions with illicit drugs in its interaction checker, ( When using the interaction checker enter the name of the medicine rather than the illicit drug.

  1. Medicines and Healthcare products Regulatory Agency. 2016. Citalopram: suspected drug interaction with cocaine; prescribers should consider enquiring about illicit drug use. URL: (accessed 19 September 2016).
  2. Lindsey WT, Stewart D, and Childress D. 2012. Drug Interactions between Common Illicit Drugs and Prescription Therapies. The American Journal of Drug and Alcohol Abuse 38(4): 334-43.
  3. Medical Council of New Zealand. 2016. Good prescribing practice. URL: (accessed 28 September 2016).
  4. Medical Council of New Zealand. 2011. Information, choice of treatment and informed consent. URL: (accessed 28 September 2016).
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