For specialist prescribers, hospital and community pharmacists, and GPs operating under shared-care arrangements. Cannabinoids interact with a wider range of medicines than is generally appreciated, both through the CYP450 system and through pharmacodynamic additive effects. This is a working checklist of the interactions and contraindications that warrant active screening in any patient being initiated on cannabis-based products for medicinal use (CBPMs), or having concomitant medicines added or removed.
Nothing here replaces a structured medication review and consultation of an authoritative drug interactions resource. The aim is to flag the categories of risk that recur most often in UK CBPM practice and to suggest a sensible documentation standard for the patient record and shared-care correspondence.
The two main mechanisms of interaction
CYP450 pharmacokinetic interactions
Both THC and CBD are metabolised by, and interact with, multiple CYP450 enzymes. The most clinically important effects are:
- CBD inhibits CYP3A4 and CYP2C19 at therapeutic doses. CYP2C19 inhibition is the more potent of the two. CBD also has weaker inhibitory effects on CYP2C9, CYP2D6 and CYP1A2.
- THC inhibits CYP2C9 moderately, and chronic THC exposure (particularly via smoking, though also relevant to chronic medicinal use) is associated with CYP1A2 induction.
- Both cannabinoids are themselves metabolised by CYP3A4 and CYP2C9; inhibitors or inducers of those enzymes can raise or lower cannabinoid exposure correspondingly.
These effects are dose-dependent and most pronounced at the higher CBD doses used in epilepsy practice, but they are clinically relevant at doses commonly used in private CBPM clinics for chronic pain, anxiety and sleep.
Pharmacodynamic additive interactions
Independently of metabolism, cannabinoids produce sedation, cognitive impairment, orthostatic effects and respiratory effects that combine additively with several other commonly prescribed medicines. The pharmacodynamic risks are easier to overlook because they do not show up on a CYP interaction checker, but in routine practice they are the more common cause of CBPM-related adverse events.
The high-risk co-medications to screen for
Clobazam
The CBD-clobazam interaction is the most thoroughly characterised cannabinoid drug interaction in the literature. CBD inhibits CYP3A4-mediated clobazam metabolism and, more potently, CYP2C19-mediated metabolism of N-desmethylclobazam, the active metabolite. Co-administration can produce three- to five-fold increases in plasma N-desmethylclobazam, with corresponding increases in sedation, ataxia and other benzodiazepine adverse effects. This is well-evidenced in the Epidyolex trial programme and in subsequent pharmacokinetic studies.
In any patient on clobazam, expect dose adjustment of the clobazam to be needed if CBD is added, and document the plan. Avoid initiating both simultaneously unless under specialist epilepsy supervision.
Warfarin and other vitamin K antagonists
Multiple case reports document supratherapeutic INR following addition of CBD or THC-containing products to stable warfarin regimens, with INR rises into the 4-7 range and corresponding warfarin dose reductions of 25-30% required. Mechanism is principally CYP2C9 inhibition by both CBD and THC, raising S-warfarin exposure. Response is not uniform — some patients show no meaningful INR shift — but the risk is high enough that any warfarin patient starting CBPM therapy needs an early INR check (within 7 days) and a tightened INR-monitoring cadence until stable.
Calcineurin inhibitors and other transplant immunosuppressants
Tacrolimus, sirolimus and ciclosporin are CYP3A4 substrates. Case reports describe substantial increases in tacrolimus trough levels in transplant patients exposed to CBD, with clinical nephrotoxicity. In transplant patients, CBPMs should be considered only under close coordination with the transplant team and with appropriate trough-level monitoring; in practice the risk-benefit calculus often argues against initiation.
SSRIs and SNRIs
SSRIs and SNRIs are commonly co-prescribed in CBPM patients given the high prevalence of anxiety and depression comorbidity. CBD’s inhibition of CYP2C19, CYP2D6 and CYP3A4 can affect concentrations of several agents in this class — escitalopram and citalopram (CYP2C19), fluoxetine and paroxetine (CYP2D6), sertraline (CYP3A4 contribution). The clinical signal is usually modest dose-shift, not catastrophic interaction, but mood and side-effect monitoring should be tightened at the point of CBPM initiation. There is also a theoretical pharmacodynamic serotonergic interaction via CBD’s 5-HT1A activity; clinically significant serotonin syndrome from this combination has not been documented but remains a sensible point of vigilance at higher CBD doses.
Benzodiazepines and Z-drugs
Additive sedation is the dominant risk. Combined with the clobazam-specific pharmacokinetic interaction, this is the most common CBPM-related cause of excessive sedation in practice. Patients on long-term benzodiazepines should have an explicit conversation about dose-stacking and a documented plan for benzodiazepine reduction once CBPM titration is stable, where clinically appropriate.
Opioids
Additive sedation and additive respiratory depression at higher opioid doses. The Project Twenty21 registry data suggests that opioid use and dose tend to fall in chronic-pain patients after CBPM initiation, which is a positive direction, but the transition phase carries real risk. Patients combining CBPMs with high-dose opioids — particularly long-acting opioids and methadone — require careful monitoring and a clear plan for opioid taper.
Stimulants and methylphenidate
CBPMs are increasingly used in patients with ADHD diagnoses. THC and stimulants have opposing acute effects on heart rate, blood pressure and arousal, with the net cardiovascular profile being unpredictable. Co-prescription is not contraindicated but warrants baseline cardiovascular assessment and explicit symptom-monitoring.
Antiepileptics other than clobazam
Beyond the clobazam interaction, CBD has documented interactions with valproate (a small increased risk of transaminitis when combined with high-dose CBD has been observed in Epidyolex trials), stiripentol, and some newer agents. Initiation of CBPMs in epilepsy patients should always be coordinated with the patient’s lead epilepsy specialist.
Other notable interactions
- Statins metabolised primarily by CYP3A4 (atorvastatin, simvastatin) may have elevated exposure with CBD. Counsel on muscle symptoms.
- Macrolide antibiotics (clarithromycin, erythromycin) inhibit CYP3A4 and can raise THC and CBD exposure. Short-course use is rarely a clinical problem; longer courses warrant dose review.
- Azole antifungals (ketoconazole, itraconazole) likewise inhibit CYP3A4 and elevate cannabinoid exposure.
- Theophylline is a CYP1A2 substrate; chronic THC use via CYP1A2 induction may lower theophylline levels.
- Anti-arrhythmics and tachycardia management: THC produces dose-dependent tachycardia. Patients on rate-control medication should be reviewed if THC-containing CBPMs are added.
Contraindications and cautions
Active or unstable cardiovascular disease
THC produces tachycardia, transient blood pressure changes and may precipitate angina in vulnerable patients. CBPMs are generally avoided in unstable angina, recent myocardial infarction, uncontrolled hypertension, decompensated heart failure and significant arrhythmias. A reasoned cardiology opinion should be obtained where the indication is otherwise strong but cardiovascular comorbidity is present.
Personal or family history of psychotic illness
This is the single most consistently emphasised caution across UK regulatory and professional bodies. THC use is associated with precipitation and worsening of psychotic symptoms in vulnerable individuals, with risk concentrated in those with personal or first-degree family history of schizophrenia or other primary psychotic disorders. The Royal College of Psychiatrists position statement (PS05/19) explicitly addresses this risk. Patients with a personal history of psychotic illness should generally not be initiated on THC-containing CBPMs; CBD-only preparations remain a more defensible option in some cases, but specialist mental-health input is appropriate.
Pregnancy and breastfeeding
CBPMs are not recommended in pregnancy or while breastfeeding. THC crosses the placenta and is detectable in breast milk; the developmental safety profile is inadequately characterised. Women of childbearing potential should be counselled on contraception during CBPM treatment and a plan documented.
Severe hepatic impairment
Cannabinoids undergo extensive hepatic metabolism. Severe hepatic impairment (Child-Pugh C) is a relative contraindication; moderate impairment warrants dose reduction and closer monitoring. Renal impairment has more modest implications for cannabinoid clearance but is relevant where comorbidity affects choice of co-medications.
Substance-misuse risk
A history of cannabis or other substance use disorder is a relative caution rather than an absolute contraindication. UK private prescribing typically requires explicit screening and a structured risk-management plan in this group. The British Pain Society position statement specifically requires assessment of prior illicit drug use as part of the pre-prescribing multidisciplinary review.
Driving: Section 5A of the Road Traffic Act 1988
Section 5A of the Road Traffic Act 1988 specifies a blood limit of 2 µg/L for THC. The Act provides a medical defence under Section 5A(3): the defence is available if the specified drug was prescribed or supplied for medical purposes, the patient took it in accordance with directions from the prescriber and the manufacturer’s accompanying instructions, and possession at the time was not unlawful under Section 5(1) of the Misuse of Drugs Act 1971. Crucially, the defence does not apply if the patient’s driving was actually impaired — Section 4 of the Act (driving while impaired through drugs) remains in play regardless of prescription status.
Practical counselling points for patients:
- Do not drive after taking a new dose of a THC-containing CBPM until they have had a settled period (typically several hours) at that dose level and feel confident they are not impaired.
- Carry evidence of prescription (a current repeat slip or letter from the clinic) in the vehicle.
- Adhere strictly to the prescribed dose and the manufacturer’s instructions — both are required to maintain the statutory defence.
- Do not combine with alcohol or other sedatives before driving.
- The defence does not apply if police can establish actual impairment. A prescription is not a licence to drive impaired.
For occupations where driving is part of the role (HGV, PSV, taxi), patients should be referred to DVLA Group 2 guidance and may need to disclose to the licensing authority. Document the conversation.
Shared-care correspondence: what to include
Where a CBPM patient has a primary-care relationship with a GP who may be asked to prescribe other medicines, a clear shared-care letter from the specialist materially reduces interaction risk. A serviceable shared-care letter covers:
- Confirmation of the specialist’s GMC Specialist Register status and the indication for CBPM use.
- Full CBPM regimen — product, formulation, daily dose, cannabinoid content.
- Concomitant medicines reviewed and judged compatible at the time of initiation.
- The specific drug-interaction categories the GP should flag to the specialist before adding new medicines (warfarin, antiepileptics, immunosuppressants, macrolides, azoles, additional sedatives).
- Driving advice given to the patient and confirmation of patient understanding.
- Review schedule and contact route for queries.
For the broader pathway context within which shared-care operates, see our overview of prescribing CBPMs on the UK specialist pathway.
Documentation in the patient record
The minimum useful record at initiation includes: indication and prior failed therapies; baseline medication list with explicit interaction screen; baseline mental-health, cardiovascular and substance-use screen; CBPM product selected with rationale; titration plan; driving advice given; consent to information sharing with GP; planned review intervals. At each review, document symptom response (ideally with validated PROs — covered in our forthcoming monitoring piece), adverse effects, dose changes and any new co-medications.
Controlled drug register entries for Schedule 2 CBPMs are required as for any Schedule 2 medicine and should be reconciled at every dispense.
A short pre-prescribing checklist
- Confirm indication and prior failed pathways.
- Run full medication list against the interaction categories above. Flag warfarin, clobazam, transplant immunosuppressants, high-dose opioids, sedatives, and SSRIs/SNRIs for explicit review.
- Screen for personal/family psychotic illness; for active or unstable cardiovascular disease; for pregnancy/breastfeeding; for severe hepatic impairment; for substance-use history.
- Counsel on driving and Section 5A medical defence requirements.
- Counsel on dose-stacking with alcohol and OTC sedatives.
- Confirm formulation choice matches clinical fit (see our companion piece on formulations).
- Issue shared-care letter to GP at first dispense.
- Set explicit review interval.
About MUZO Health
MUZO Health was created to raise the standard of cannabis-based medicines in the UK, with a focus on quality, consistency and clinical integrity. Our mission is to support clinicians, empower patients and help move cannabis medicine forward responsibly, transparently and without compromise. Learn more about MUZO Health.