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Since cannabis was partly legalised in Germany, it has become part of everyday life for many adults — and it still rarely comes up in the consulting room. That is understandable and it is still a problem, because cannabis interacts with medicines along two very concrete routes: it amplifies sedating effects, and individual constituents slow down the liver enzymes that break down many medicines. This article sets out matter-of-factly where that becomes relevant — without moralising and without playing anything down.
brite shows you which of your medicines reinforce one another.
Cannabis often goes unmentioned at the appointment: for fear of being judged, of a note in the records, of consequences. The worry is human — but it costs exactly the piece of information that safe treatment depends on.
Four pieces of information are genuinely useful, and you can give all of them without any long explanation:
If saying it out loud is hard, write it on your medication list. How to keep such a list complete — including the things that come without a prescription — is covered in the guide Keeping a medication list.
Almost every relevant interaction comes back to one of two principles. Once you know them, you can judge for yourself when a combination deserves attention.
| Mechanism | What happens | Typically affected | What to watch for |
|---|---|---|---|
| Additive sedation (pharmacodynamic) | Two substances depress the central nervous system — the effects add up | Benzodiazepines, opioids, sleeping tablets, sedating antidepressants and antihistamines, alcohol | Tiredness, grogginess, risk of falls, impaired fitness to drive, in rare cases respiratory depression |
| Enzyme inhibition in the liver (pharmacokinetic) | CBD above all inhibits breakdown enzymes (the CYP system) — other substances are cleared more slowly and their levels rise | Anticoagulants, individual antiepileptics, some statins, other substances broken down via CYP | Amplified effects and side effects, altered laboratory values (e.g. INR, liver values) |
There is a third point on top of that, which is not an interaction in the narrow sense: cannabis acts on the circulation, the airways and the mind — and it meets people who already have a condition in exactly those places.
This is the mechanism with the greatest everyday relevance, because you notice it straight away and because it affects the most people.
Sedatives such as lorazepam and sleeping tablets such as zolpidem depress the central nervous system. Together with THC, tiredness, slowed reactions and gaps in memory become more pronounced. In older people the risk of falls comes on top, which in that age group is one of the most relevant avoidable risks there is. What otherwise applies to sleeping tablets — including the alternatives — is set out in the guide Sleeping pills: what really helps.
With opioid-type painkillers such as tramadol or tilidine, the combination is critical for two reasons: sedation and grogginess add up, and at higher doses opioids suppress the drive to breathe. The evidence for clinically meaningful respiratory depression from cannabis alone is limited — but in combination with opioids, and all the more so alongside alcohol or benzodiazepines, caution is warranted. Anyone taking opioids regularly should therefore raise their cannabis use, not least because it changes how the pain treatment is judged. The painkillers compared guide puts the drug groups into context.
Mirtazapine, amitriptyline and quetiapine make you tired too. The combination rarely leads to dramatic events, but it often leads to a state in which you simply cannot wake up in the morning — and that is then wrongly put down to the medicine alone.
Alcohol is the most frequent companion and it amplifies both sides. The combination of alcohol, a sedating medicine and cannabis is the riskiest of the constellations discussed here — particularly where coordination, judgement and vomiting while asleep are concerned. The basics are in the guide Medications and alcohol.
The time axis matters too: THC stays detectable in the blood far longer than the noticeable high lasts — with regular use possibly for days. "I feel clear again" is therefore not a reliable criterion for the legal side. For actual fitness to drive the reverse applies: even without feeling high, residual effects and the amplification by medicines can persist.
The second mechanism is invisible — and that is exactly what makes it treacherous. In the liver, an enzyme system (cytochrome P450, CYP for short) breaks down the great majority of all medicines. CBD inhibits several of these enzymes, in particular CYP3A4, CYP2C19 and CYP2C9. If an enzyme is slowed, the substance that depends on it is removed from the body more slowly — its level rises, and with it the effect and the side effects. THC itself is also metabolised via CYP2C9 and CYP3A4 and can intervene in this interplay.¹
The principle is the same as in the well-known grapefruit effect, only with a different cause — as described in the guide Grapefruit and medications.
Two qualifications belong to an honest account: the extent depends heavily on the dose — a few drops of CBD oil are something quite different from the high doses used in drug trials. And for many individual combinations the evidence is thin. The conclusion is therefore not "dangerous" but "observe and monitor".
With vitamin K antagonists the link is described most clearly: for warfarin there are case reports of a marked rise in the INR under CBD. For phenprocoumon, the one commonly used in Germany, the same breakdown route is involved, so a comparable effect is plausible.² In practice that means: anyone taking phenprocoumon who uses CBD needs closer INR monitoring — and should treat starting, stopping or changing the dose of a CBD product as what it is: a change of treatment. What else counts in everyday life with anticoagulants is covered in the guide Living with blood thinners.
With epilepsy the matter is doubly delicate, because drug levels have to be kept particularly precisely. The best studied interactions of CBD are those with clobazam and valproate. For other substances such as lamotrigine, which is largely broken down by a different route, the evidence is limited — which does not mean that nothing happens, only that it cannot be predicted. With epilepsy the rule is therefore: never use cannabis without the knowledge of the treating practice, because level checks are part of the package and because lack of sleep and breaks in use can additionally affect the seizure threshold.
Cholesterol-lowering drugs such as simvastatin are broken down via CYP3A4. A raised level typically shows up as muscle pain — precisely the symptom that people on statins often report anyway. The clinical significance is not well established; even so, if you develop new muscle problems and use CBD, mention it.
brite checks your combinations and makes the overview for your appointment easy.
THC has short-term effects on the cardiovascular system: the pulse often rises markedly in the first hour after use, and blood pressure can go up at first and then drop on standing. For healthy young people that usually has no consequences. With pre-existing heart disease it does not necessarily stay that way.
Smoking cannabis — particularly mixed with tobacco — produces combustion products that irritate the airways. Chronic cough, phlegm and a raised susceptibility to bronchitis are described. For people with asthma or COPD that matters, because it puts additional strain on the underlying condition — and because any deterioration is then wrongly attributed to the maintenance treatment.
Anyone using inhaled medicines such as salbutamol or budesonide should bear two things in mind. The maintenance treatment is not replaced or made superfluous by acute symptoms easing off briefly after use. And reaching for the reliever inhaler more often is a warning sign, not a normal operating state. On correct technique, see the guide Inhaling correctly.
Vaporising avoids combustion and is regarded in this respect as less of a burden than smoking — it is not burden-free, and long-term data are limited. That is an honest answer, not a recommendation.
This section needs both: precision and restraint. Cannabis is often used to relax, against rumination or to get to sleep — and for many people that works in the short term. Where a mental health condition already exists, the picture is more complicated.
Stopping matters too: anyone who uses cannabis daily and then stops often experiences disturbed sleep, inner restlessness and irritability for several days to weeks — easily confused with a worsening of the underlying condition or with the withdrawal symptoms of a medicine, see Stopping SSRIs.
Medical cannabis is something other than recreational use — not morally, but pharmacologically. In Germany it is prescribed by a doctor and dispensed by a pharmacy; the products are subject to quality requirements, the THC and CBD content is declared, and it is used according to a dosing schedule, usually built up gradually. It is used among other things for certain chronic pain syndromes, for spasticity in multiple sclerosis, and for loss of appetite and nausea in particular situations. The strength of the evidence differs by indication and remains limited for some uses — that too belongs to an honest account.⁴
The decisive difference for this topic: with a prescription, the interactions have been thought through, the dose is known and the checks are planned in. With recreational use all of that is missing — the content, the amount and the accompanying substances are as a rule unknown. That is precisely why telling the practice yourself matters so much: it replaces the information that a prescription would have provided anyway.
The two mechanisms from section 2 apply unchanged to prescribed medical cannabis as well. Anyone taking it should therefore keep it on the medication plan like any other medicine — see Drug interactions.
Here the recommendation is unambiguous and not a matter of weighing things up: in pregnancy and while breastfeeding, cannabis in every form is advised against — THC-containing as well as CBD-containing, smoked, vaped or eaten. THC crosses the placenta and passes into breast milk; indications of effects on growth and on the child's development are reason enough for clear restraint, even though the evidence is limited for obvious reasons. If you are pregnant and use cannabis regularly, say so openly — this is about support, not reproach. The general principles for handling medicines during this time are set out in the guide Medications during pregnancy.
CBD oils, capsules and drops are sold outside pharmacies and many people do not perceive them as an "active substance" at all. That is exactly where the problem lies: they appear on no medication plan — and yet they act on the same enzyme system as some prescription-only medicines. On top of that, the actual content of freely available products can vary.
The conclusion is simple: CBD belongs on the medication list — just like St John's wort, high-dose vitamin D or magnesium. Why over-the-counter preparations are so often overlooked is explained in the guide Supplements and medications.
The interaction check shows you the risks in seconds — without judgement.
Most combinations lead to nothing more dramatic than tiredness. But there are situations in which people wait when they ought to act.
You should get medical advice promptly for: newly occurring palpitations, unusually heavy bruising or bleeding while on anticoagulants, suddenly altered laboratory values, new muscle pain on a statin, or a marked worsening of a mental health condition.
And one sentence for any emergency: tell the ambulance crew what has been taken. That determines the right treatment. Staying silent out of fear puts you, or the person this is about, at risk. See also Medication side effects.
Interaction check
Checks your combinations — and shows you above all the sedating substances that reinforce one another.
Digital medication plan
Every preparation in one place, including over-the-counter CBD products — a clean basis for your next appointment.
Health history
Symptoms and laboratory values can be recorded alongside — so it becomes clear to follow whether anything has really changed since an adjustment.
