Paranoia Explored in Cannabis Study

Recent findings are helping to explain cannabis’s paranoia-inducing effects.

The main psychoactive ingredient in cannabis is delta-9-tetrahydrocannabinol, or THC. It is well known that THC impairs cognitive functioning. But studies of other effects on the brain have yielded conflicting results.

Professor Daniel Freeman of Oxford University, U.K., and colleagues tested the effects on 121 participants aged 21 to 50, who had all taken cannabis at least once prior to the study.

None had a history of mental illness but all reported occasional “mistrustful thinking” in their day-to-day lives. This is to be expected, as previous surveys have shown that about half of all individuals had paranoid-type thoughts in the past month.

THC injections “equivalent to a strong joint” were given to two-thirds of the participants, while the remaining third were injected with a placebo. Injections were used to reach a reliable THC level in the bloodstream.

Next, a range of tests were carried out to measured excessive suspiciousness, such as real-life social situations, a virtual reality simulation, self-report questionnaires, and clinical interviews.

Those given THC were more likely to report paranoid thoughts, a rate of 50 percent versus 30 percent on placebo. The researchers say this translates as an increase in paranoia directly attributable to the THC among 20 percent of the participants. Paranoid feelings declined as the drug left the bloodstream over the subsequent 90 minutes.

A range of other psychological effects were recorded, including anxiety, worry, lowered mood, negative thoughts about the self, various changes in perception such as sounds being louder than normal and colors brighter, thoughts echoing, altered perception of time, and poorer short-term memory.

Freeman said, “Paranoia is excessive thinking that other people are trying to harm us. It’s very common because in our day-to-day lives we have to weigh up whether to trust or mistrust, and when we get it wrong, that’s paranoia.

“Many people have a few paranoid thoughts, and a few people have many paranoid thoughts.”

In the Schizophrenia Bulletin, the team states that this represents the largest and most in-depth study of the effects of THC.

“The study very convincingly shows that cannabis can cause short-term paranoia in some people,” they report. “But more importantly this study 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.”

Freeman said the study identifies a number of highly plausible ways in which the mind promotes paranoid fears. “Worry skews our view of the world and makes us focus on perceived threat. Thinking we are inferior means we feel vulnerable to harm. Just small differences in our perception can make us feel that something strange and even frightening is going on.”

Although the study provided useful information on the immediate effects of cannabis, it did not aim to investigate clinically severe paranoia. Beyond the implications for cannabis users, it highlights the basis of normal paranoid-type thoughts that affect everyone.

“Reducing time spent ruminating, being more confident in ourselves, and not catastrophizing when unusual perceptual disturbances occur will in all likelihood lessen paranoia,” Freeman said.

The role of self-confidence in protecting against paranoia was explored further in a study by psychiatrists led by Professor Graham Dunn of Manchester University, U.K. His team used a virtual reality environment to investigate the impact of negative thoughts about the self on tendency for paranoia.

The 26 men who took part were more likely to express paranoia about their virtual reality avatars when their self-confidence was negatively manipulated beforehand.

“Low self-confidence manipulation, relative to the high self-confidence manipulation, led to significantly more negative social comparison in virtual reality and higher levels of paranoia,” the authors reported in the journal Behavioural and Cognitive Psychotherapy.

They recommend that interventions designed to improve self-confidence are used to reduce paranoia, or “persecutory ideation.”

Another team, also from Oxford, investigated effective methods of ending such persecutory thoughts. They recruited eight patients with paranoid thoughts and high levels of worry.

From semi-structured interviews, several themes were identified as important for bringing worry episodes to a close. These included distraction, interpersonal support, and reality testing.

“Patients with persecutory delusions report worry being uncontrollable and distressing but are able to identify ways that a period of worry can stop,” reported the team. “The study suggests that building on individuals’ distraction techniques, reality-testing ability and their social support network could be of benefit.”


Freeman, D. et al. How cannabis causes paranoia: Using the intravenous administration of delta-9-tetrahydrocannabinol (THC) to identify key cognitive mechanisms leading to paranoia. Schizophrenia Bulletin, 16 July 2014.

Atherton, S. et al. Self-Confidence and Paranoia: An Experimental Study Using an Immersive Virtual Reality Social Situation. Behavioural and Cognitive Psychotherapy, 11 November 2014, doi: 10.1017/S1352465814000496

Startup, H. et al. How Do Individuals with Persecutory Delusions Bring Worry to a Close? An Interpretive Phenomenological Analysis. Behavioural and Cognitive Psychotherapy, 31 January 2014, doi: 10.1017/S1352465813001136

Canibis brain photo by shutterstock.