Taking cannabis can affect people's anxiety levels. Long-term use has been associated with anxiety symptoms, panic attacks and an increased risk of anxiety disorders and sudden and severe increases in anxiety can also occur following cannabis use. However, cannabis use can also lead to sedation and relaxation and many people who take the drug say it makes them feel less anxious. A team of researchers at the Institute of Psychiatry in London gave 15 healthy volunteers (none of whom were regular cannabis users) two chemicals found in cannabis, 9-THC and cannabidiol. They then showed them faces, some of which looked frightened and distressed, designed to make them feel anxious. The participants' anxiety levels were measured by changes in their skin using the same techniques as lie-detectors, and they also underwent a brain scan. 9-THC was found to increase anxiety as well as levels of intoxication, sedation and psychotic symptoms. Cannabidiol reduced anxiety. The two chemicals were also found to affect different parts of the brain. 9-THC affected the frontal and parietal parts of the brain whereas cannabidiol affected the amygdala and the anterior and posterior cingulate cortex.
Fusar-Poli, Paolo ... [et al] - Distinct effects of 9-tetrahydrocannabinol and cannabidiol on neural activation during emotional processing Archives of General Psychiatry January 2009, 66(1), 95-105