CBD & medications – interactions

TL;DR

• CBD inhibits CYP3A4, CYP2C19, CYP2C9 – like grapefruit.
• Critical: warfarin (INR monitoring), clobazam, tacrolimus.
• Threshold: 20 mg/kg CBD (Epidyolex) → +300 % clobazam levels.
• Tell your GP/pharmacist before starting if on chronic meds.

Mechanism – why CBD interacts

CBD is a potent inhibitor of several cytochromes (CYP3A4, CYP2C19, CYP2C9, CYP2D6). Around 60 % of drugs authorised in Switzerland are metabolised through one of them – similar to the classic grapefruit effect. Clinical interactions are well documented: Devinsky 2017 (NEJM) for clobazam and Brown/Winterstein 2019 for warfarin, tacrolimus.

Red list – always talk to your doctor

• Anticoagulants: warfarin (Marcoumar), sometimes apixaban.
• Antiepileptics: clobazam, valproate, phenytoin, topiramate.
• Immunosuppressants: tacrolimus, ciclosporin, sirolimus.
• Antidepressants: SSRIs (sertraline, citalopram), tricyclics.
• Cardio: amiodarone, some statins (simvastatin).
• Opioids (fentanyl, oxycodone) – additive sedation.

Concrete risk classes and what to discuss with your doctor

CBD mainly inhibits CYP3A4 and CYP2C19, plus CYP2C9 in a dose-dependent way. The relevant classes are anticoagulants (warfarin, partly DOACs), antiepileptics (clobazam, whose active metabolite N-desmethylclobazam can triple), immunosuppressants (tacrolimus, ciclosporin), some statins and proton-pump inhibitors. Clinical relevance usually begins around 10 mg/kg body weight, as in the Epidyolex trials; everyday doses of 20–50 mg are rarely an issue for healthy adults without chronic medication. If you are on long-term medication, bring the exact daily CBD dose, the product and the COA to the appointment – that lets a physician target plasma-level monitoring instead of advising against it outright.

Swiss Classification: Medical Liability and Pharmacy Consultation

Since the amendment to the law in August 2022, medical cannabis can be prescribed in Switzerland without an exceptional permit from the Federal Office of Public Health (BAG). This has placed the responsibility directly in the hands of Swiss physicians and pharmacies. When prescribing high-dose CBD or advising on over-the-counter CBD products, professionals must systematically weigh the risk of interactions. The Swiss Society for Cannabis in Medicine provides guidance on this, as unforeseen interactions can raise liability issues if adequate information has not been provided.

In Swiss pharmacy practice, the dispensing of CBD preparations is increasingly recorded digitally in order to cross-reference interaction checks with the existing regular customer database. However, since many consumers view CBD as a harmless food supplement and conceal its use from their doctors, a high number of undetected interactions remains unreported. Swiss professional associations therefore recommend proactive history-taking. When purchasing prescription medications at the pharmacy, consumers should always disclose whether they regularly take CBD oils or other hemp products in order to proactively minimize health risks in everyday life.

• Personal responsibility of the Swiss medical profession since the liberalisation in August 2022.
• Importance of seamless documentation in the pharmacy's customer records for interaction checks.
• Liability relevance in the case of insufficient education on enzymatic interactions.
• Necessity of proactive disclosure of CBD use by patients.

Practical monitoring and laboratory controls in everyday life

If the combination of CBD and a critical medication is medically necessary or unavoidable, therapeutic drug monitoring (TDM) comes into focus. In this process, blood samples are taken regularly to determine the exact concentration of the medication in the blood plasma. Swiss laboratories can perform these analyses precisely, allowing the dose of the primary medication to be adjusted step-by-step if necessary. This procedure prevents the enzymatic blockade of the liver from causing a gradual overdose and thus toxic side effects in the body.

In addition to pure active substance levels in the blood, regular checks of liver values are a central component of clinical monitoring. Since both CBD and many concomitant medications are metabolised by the liver, a continuous double burden on this organ can lead to an elevation of liver enzymes. At the first signs of fatigue, loss of appetite, or altered laboratory values, the dosage must be reviewed immediately in consultation with the treating physician. Unauthorised discontinuation or reduction of the prescribed medication must be avoided under all circumstances.

• Therapeutic drug monitoring (TDM) for the precise determination of active substance concentrations in blood plasma.
• Regular laboratory checks of liver values for the early detection of organ strain.
• Individual dose adjustment of the primary medication by medical professionals.
• Avoidance of health risks through structured, medically supervised monitoring intervals.

Influence of the dosage form on the risk of interaction

The risk of clinically relevant interactions depends significantly on how CBD is administered to the body. With oral ingestion, for example in the form of capsules, tablets, or enriched foods, the cannabinoid passes through the gastrointestinal tract and enters the liver directly via the portal vein. This so-called first-pass effect means that a large amount of CBD directly encounters and blocks the cytochrome P450 enzymes there. The probability of interactions with concomitantly taken medications is highest with this form of consumption.

The situation is different with sublingual absorption via the oral mucosa or inhalation by vaporisation. In these cases, part of the active ingredient enters the systemic circulation directly, thereby partially bypassing the first pass through the liver. Nevertheless, caution is also advised here, as the CBD must ultimately be metabolised. In contrast, topical applications such as CBD creams or ointments for local application on the skin exhibit very low systemic bioavailability. With purely external application, the risk of systemic interactions with oral medications is negligibly low based on current knowledge.

• High risk of interaction with oral ingestion due to the pronounced first-pass effect in the liver.
• Partial bypassing of the first liver passage with sublingual and inhalative use.
• Very low systemic absorption and minimal risk with topical CBD skincare products.
• Importance of choosing the route of administration for risk minimisation in daily therapeutic practice.

Dangers of contamination and unclear product declarations

An often underestimated risk in assessing interactions lies in the lack of standardisation and quality of over-the-counter CBD products. Numerous preparations exist on the Swiss market whose actual cannabinoid content deviates significantly from the information on the label. If a product unnoticedly contains significantly more CBD than declared, the potential for enzyme inhibition increases unpredictably. In addition, contamination with heavy metals, pesticides, or solvents can place an additional burden on the liver and influence the metabolism of medications in unpredictable ways.

The presence of undeclared traces of THC is particularly critical. Since products with a total THC content of less than one percent may be legally sold in Switzerland, many full-spectrum oils contain relevant amounts of this psychoactive cannabinoid. THC is metabolised via similar enzymatic pathways as CBD and can further increase or alter interactions with medicinal products. For patients, it is therefore advisable to rely exclusively on certified products with independent laboratory analyses in order to know the exact composition and avoid uncontrolled risks.

• Unpredictable interaction risks due to discrepancies between declared and actual CBD content.
• Additional burden on the liver due to potential contaminants such as pesticides or heavy metals.
• Amplification of interactions due to legally tolerated traces of THC in Swiss full-spectrum products.
• Necessity of independent certificates of analysis (COAs) to ensure product safety.

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