Cannabis, polypharmacy and dangerous interactions

TL;DR

• THC and CBD inhibit CYP3A4/CYP2C9 — pathways central to many age-related medications.
• Warfarin/DOACs: increased bleeding risk, INR monitoring needed.
• Statins like simvastatin/atorvastatin: increased blood levels, muscle risk.
• Benzodiazepines + cannabis: additive sedation, higher fall risk.

The pharmacokinetic mechanism

THC and CBD are mainly metabolised via CYP3A4 and CYP2C9 and themselves act as inhibitors of these enzymes. When medications degraded by the same enzymes are taken concurrently, their plasma concentration rises — a classic pharmacokinetic interaction mechanism, especially relevant in seniors whose liver capacity is already age-reduced (van den Bogaard 2023).

Practical consequences for care

Before starting cannabis therapy, the full medication list should be reviewed by a physician or pharmacist. For warfarin, closer INR monitoring in the first weeks is sensible; for statins, a dose adjustment or switch to a statin with a different metabolic pathway (e.g. pravastatin, less CYP3A4-dependent) may be considered.

Pharmacodynamic interactions in old age

In addition to biochemical metabolic pathways in the liver, direct interactions in the central nervous system play a significant role. When elderly people take cannabis together with other depressant substances, the effects can reinforce each other. This particularly affects medications for sleep disorders, strong painkillers, or antidepressants. The combination often leads to pronounced fatigue, poor concentration, and a reduced reaction speed. In daily life, this increased sedation significantly increases the risk of serious falls and cognitive impairment in the home environment.

Another risk affects the cardiovascular system, which in old age often reacts less flexibly to fluctuations in blood pressure. Cannabinoids can affect blood pressure and lead to a sudden drop in blood pressure, especially when standing up. If antihypertensive drugs or diuretics are taken at the same time, this orthostatic effect is amplified. Those affected then suffer from dizziness, vision blacking out, or brief loss of consciousness. Careful coordination of intake times and regular monitoring of vital signs by medical professionals are therefore essential.

• Enhancement of the sedating effect when taken concurrently with sleeping pills or opioids.
• Increased risk of orthostatic hypotension and resulting fall injuries.
• Influence on cognitive performance and general responsiveness in daily life.
• Need for close monitoring of blood pressure and pulse rate in combined use.

The role of CBD products from the Swiss retail sector

In Switzerland, CBD products with a total THC content of less than one percent are legal and freely available in the retail sector and in drugstores. Many older people use these oils, capsules or teas for self-medication for sleep problems or chronic pain without discussing this with their family doctor. Since these products are not declared as therapeutic products, they are subject to less stringent medical controls than prescription preparations. Nevertheless, over-the-counter CBD contains highly active substances that strongly influence liver enzymes and can thus unpredictably alter the effects of other prescribed medications.

The competent federal offices, such as the BAG and the BLV, point out that even over-the-counter hemp products are not harmless food supplements. Since CBD is a potent inhibitor of certain enzyme systems, uncontrolled intake can lead to dangerous accumulations of cardiac medications or blood thinners in the body. For senior citizens, it is therefore advisable to always view even supposedly gentle natural products from the Swiss retail sector as an active part of their medication and to disclose their consumption at the pharmacy or medical practice.

• Over-the-counter CBD products in Switzerland are subject to the limit of less than one percent total THC.
• CBD blocks important metabolic pathways in the liver and compromises the dosing accuracy of cardiac medications.
• A lack of declaration requirements in the retail sector makes precise dose finding difficult for patients.
• Consultation in Swiss pharmacies protects against undesirable interactions with over-the-counter hemp products.

Medical supervision and prescribing practice in Switzerland

Since the amendment to the law in August 2022, the prescription of medical cannabis in Switzerland has been significantly simplified. Doctors can now prescribe corresponding preparations directly, without having to obtain a time-consuming exceptional permit from the Federal Office of Public Health (BAG). This liberalisation considerably facilitates access for elderly patients suffering from pain and spasticity. At the same time, it places a great responsibility on the medical profession, as the prescription must now be accompanied by a systematic recording of all medication and an individual risk assessment.

Within the framework of the controlled dispensing of medical cannabis, which is also scientifically monitored via specialised pilot trials in various Swiss cities, patient safety is the top priority. Swissmedic monitors the quality of the authorised formulations, which guarantees a constant dosage compared to unregulated products. For older people with polypharmacy, this official route offers the decisive advantage that interactions can be detected at an early stage through accompanying pharmacovigilance and targetedly countered by adjusting the accompanying medication.

• Direct medical prescribability of medical cannabis in Switzerland established since August 2022.
• Quality assurance and standardisation of active substance contents under the supervision of Swissmedic.
• Scientific findings from ongoing Swiss pilot trials on the controlled dispensing of cannabis.
• Avoidance of dosage fluctuations compared to illegal or unregulated market sources.

Risk minimization and monitoring in everyday life

To minimize the risk of interactions in the case of existing polymedication, a structured approach in everyday life is essential. Seniors should maintain an up-to-date medication plan on which all medically prescribed drugs as well as all self-purchased cannabis and CBD products are fully recorded. With every new prescription or dose adjustment of a medication, it must be checked whether the rate of degradation in the body changes. A gradual dose adjustment of cannabis, starting with very small amounts, helps to safely test tolerability in individual cases.

If unexpected symptoms such as extreme fatigue, dizziness, confusion, or unexplained bruising occur during combined use, medical advice must be sought immediately. In urgent suspected cases of poisoning or severe intolerance, Tox Info Suisse provides a fast and competent point of contact for those affected and their relatives via the emergency number 145. Awareness of these warning signs and an open dialogue with the treating healthcare professionals form the most important safety net for low-risk use in old age.

• Maintaining a complete Swiss medication plan, including all herbal preparations.
• Gradual titration of cannabinoids for the early detection of individual intolerances.
• Immediate contact with Tox Info Suisse via the emergency number 145 in the event of acute poisoning symptoms.
• Regular laboratory checks of liver and kidney values to monitor organ functions in old age.

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