How Cannabis Causes Paranoia

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Cannabis study provides insight into how to treat serious mental disorders.

Cannabis causes short-term paranoia and it’s not related to memory problems, a comprehensive new study has found.

The researchers discovered that after being given the main psychoactive ingredient in cannabis, THC (Δ9-tetrahydrocannabinol), participants became anxious, developed low self-esteem and experienced unsettling changes to their perceptions (Freeman et al., 2014).

The study’s lead author, Professor Daniel Freeman, said:

“The study very convincingly shows that cannabis can cause short-term paranoia in some people.

But more importantly it shines a light on the way our mind encourages paranoia.

Paranoia is likely to occur when we are worried, think negatively about ourselves, and experience unsettling changes in our perceptions.”

There were 121 people taking part in the study, none of whom were suffering from mental illness, but who had used cannabis before.

Two-thirds of them were injected with THC, while the remainder were injected with a placebo.

The amount was equivalent to a strong joint.

Half the people who were given the drug experienced paranoid thoughts, compared with 30% in the control group.

On top of this there were a variety of other psychological effects of the THC:

  • thoughts echoing,
  • altered perception of time,
  • anxiety,
  • various changes in perception such as sounds being louder than normal and colours brighter,
  • lowered mood,
  • negative thoughts about the self,
  • and poorer short-term memory.

When they looked at the results statistically, they were able to estimate what was causing the paranoia.

It turned out that it wasn’t a poorer memory:

“The increase in negative affect and in anomalous experiences fully accounted for the increase in paranoia.

Working memory changes did not lead to paranoia.

Making participants aware of the effects of THC had little impact.”

The scientists think the study of how cannabis causes paranoia will help the treatment of mental disorders which included delusional states, like schizophrenia:

“The clear clinical implication is that reducing negative emotion in patients with delusions, eg, by reducing the tendency to worry, testing out anxious fears, and increasing self-confidence, will lead to improvements in paranoia.

Also, the identification, normalization, and reduction of subtle anomalies of experience (eg, by reducing triggers and learning to tolerate the confusing sensory experiences) are clinically warranted.” (Freeman et al., 2014).

Fascinatingly, some people in the control group who were given a placebo also acted stoned, and it was difficult to tell the difference Freeman explained:

“…the placebo produced extraordinary effects in certain individuals.

They were convinced they were stoned, and acted accordingly.

Because at the time we didn’t know who had been given the drug, we assumed they were high too.”

Image credit: Alistair Holmes

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About the author


Dr Jeremy Dean is a psychologist and the author of PsyBlog. His latest book is "Making Habits, Breaking Habits: How to Make Changes That Stick". You can follow PsyBlog by email, by RSS feed, on Twitter and Google+.

Published: 22 July 2014

Text: © All rights reserved.

Images: Creative Commons License