Who should avoid cannabis
Pregnant, breastfeeding or trying to conceiveTHC reaches the child via the umbilical cord and breast milk.
Psychosis in the familyParents or siblings with psychosis.
Heart or circulatory diseasePulse and blood pressure rise straight after use.
Under 25The brain is still maturing.
AddictionIncluding a past one: risk of switching substances or relapsing.
Regular medicationInteractions, for example with antidepressants or anticoagulants.
Pregnancy, breastfeeding and trying to conceive
THC reaches the unborn child via the umbilical cord. The child’s organs are not yet developed enough to protect it. Smoking cannabis during pregnancy is linked to lower birth weight; a large review also found higher risks of premature birth and growth restriction. Whether children are later affected in learning or attention is still an open question.
No safe amount is known. The recommendation is therefore to avoid cannabis completely during pregnancy, and likewise while breastfeeding: THC passes into breast milk, and in one study it was detectable for up to six days. Anyone who wants to have a child is also better off avoiding it, because cannabis can alter the hormone balance and reduce fertility.
Psychosis in the family and mental illness
Someone is considered to have a family history if their parents or siblings have had psychosis. For these people, the recommendation is to avoid cannabis. Anyone who has already had a psychosis risks a poorer recovery and more relapses by using it. In bipolar disorder, regular use goes hand in hand with more manic symptoms. People with anxiety or depression also have a higher risk of psychosis.
Heart and circulation
Pulse and blood pressure can rise straight after use. An increased risk of heart attack is assumed for the first hour afterwards, and with pre-existing conditions the heart can be overloaded. Large studies find more cardiovascular events in people who use cannabis; this does not yet prove that cannabis is the cause.
Young people
The brain matures until the mid-20s, and the risk of dependence and psychosis is higher. More on this in the module Why age matters.
Addiction
Cannabis use is also linked to developing dependence on alcohol, tobacco and other drugs. Anyone who has or has had an addiction risks switching to another addictive substance; this is considered a common trigger for relapse.
Medication
THC and CBD inhibit enzymes in the liver that break down many medicines. As a result, antidepressants, for example, can have a stronger effect, and with anticoagulants the risk of bleeding can rise. The data mostly come from the laboratory. How strong the effect is in everyday life is unclear. If you take medication regularly, it is best to discuss this with your doctor or pharmacist.
Alcohol and cannabis together
Together, the effects reinforce each other: stronger light-headedness, confusion, nausea and vomiting, up to loss of control. The body’s warning signals are noticed less. Combined use is considered particularly dangerous.
Sources
- familienplanung.de (BIÖG): Drogen in der Schwangerschaft
- DigiSucht (2026): Cannabis in der Schwangerschaft und Stillzeit
- Hessisches Ärzteblatt 9/2025: Cannabiskonsum in der Schwangerschaft
- CDC (2024): Cannabis and Breastfeeding
- drugcom.de (BIÖG, 2023): Empfehlungen zur Reduzierung des Psychose-Risikos
- cannabispraevention.de (BIÖG): Faktoren, die das Gesundheitsrisiko erhöhen
- drugcom.de (BIÖG, 2023): Cannabis mit erhöhtem Risiko für Herz-Kreislauferkrankungen verbunden
- National Academies of Sciences (2017): The Health Effects of Cannabis and Cannabinoids
- Pharmazeutische Zeitung (2022): Cannabis als Interaktionspartner
- drugcom.de (BIÖG, 2017): Mehr unerwünschte Effekte bei Mischkonsum von Alkohol und Cannabis
- NIDA (2024): DrugFacts Cannabis