How solid is the research on psychosis risk?

The research on the risk of psychosis from cannabis is extensive but inconclusive. Most studies show a link between use and psychotic symptoms. However, they usually cannot prove that cannabis is the cause. This article explains what studies can achieve, which findings are cited most often and how to classify them.

What a study can show

Observational studies compare people who use cannabis with people who do not, and look at how often certain illnesses occur. Such studies show links. However, they do not automatically show that one factor causes the other.

Cannabispraevention.de states that this statistical link does not prove that cannabis actually causes psychosis. Drugcom writes that research increasingly suggests cannabis can trigger a schizophrenia that was previously hidden. According to drugcom, this is not yet proven beyond doubt.

Findings on THC content

An analysis that drugcom summarises reports roughly double the risk of psychotic symptoms with high-strength cannabis. High-strength means that the THC content is higher than in weaker products. The finding matters for prevention because it shows that strength plays a role as well as frequency.

The drugcom drug encyclopedia also describes the psychosis risk as particularly high for people who mainly use high-strength varieties.

Findings on young people

For young people, drugcom reports a study according to which 12- to 19-year-olds who use cannabis have a roughly elevenfold higher risk of a psychotic disorder. This figure comes from a single analysis and should not be read in isolation. It does, however, emphasise that young people are especially at risk.

Prevention adds that the harmfulness of cannabis depends on age. The young brain is in an important stage of development, and THC can disrupt maturation processes in the brain.

The limits of research

Studies cannot account for all influences at the same time. Family, friends, sleep, stress and pre-existing conditions interact. One study can adjust for some of these, but rarely for all. Some uncertainty therefore always remains.

The selection of participants also matters. People who volunteer for a study or are treated in a clinic may differ from the general population. This can shift results.

Why headlines can mislead

A figure such as double or elevenfold risk sounds definite. Without context, it seems like a fixed quantity. Relative values do not say on their own how many people are actually affected.

This is why every report deserves a second look: who conducted the research, with how many people, over what period and with which method?

How to classify studies

• Type of study: Observation shows links, and a cause is hard to prove with it.
• Size: Small studies are less meaningful than large ones.
• Comparison group: How were people without use selected?
• Absolute numbers: Ask how many people are actually affected.
• Source: Does the report come from a specialist agency or from an article that sensationalises it?

What this means for people who use cannabis

The research calls for caution, though it does not justify alarm. If you take the risk of psychosis seriously, pay attention to the age, frequency and strength of your use. If psychosis runs in your family, speak with a doctor.

For people who already experience problems, use is a factor to keep in view. If you recognise yourself in this, an addiction counselling service can help you assess your situation.

Prevention: knowledge protects

People who can classify findings make better decisions. Young people in particular are often confronted with exaggerated reports. Sober classification helps you understand the risks without being guided by either panic or playing them down.

Section 5(1) of the Cannabis Act (KCanG) prohibits use in the immediate presence of persons under 18, and possession by persons under 18 is not permitted under section 3 KCanG. These rules add a legal framework to prevention.

Contact points

For questions about your own use, you can reach the Sucht & Drogen Hotline on 01806 313031. A local addiction counselling service also offers advice. For health questions, a doctor is the right contact.

When a study matters

A single study is rarely decisive. A finding becomes important when it appears in several studies that use different methods. Then it is worth taking the results seriously, even if they are not yet regarded as conclusive.

Common misunderstandings

• Association equals cause: Anyone who equates the two overestimates the finding.
• One average applies to everyone: Individual predisposition and living circumstances change the picture.
• A number is a verdict: Numbers need context, otherwise they say little.

Further reading: Can cannabis cause schizophrenia?, Which pre-existing conditions raise the risk of psychosis? and What role does THC content play in risk?

What you can do now

1. Ask about source, sample size and absolute numbers when you see a report.
2. Pay attention to the age, frequency and strength of your own use.
3. If psychosis runs in your family, speak with a doctor.
4. Seek counselling if you are unsure how to classify your use.