Why does marijuana cause anxiety? Marijuana is getting increasingly used in medical settings. However, one of its disadvantages is anxiety. Does anyone know the reason behind the anxiety? Is it from the marijuana itself, or does it result secondarily due the effects of the marijuana, because, e.g. the user is not comfortable with the effects?
Cannabis can cause an acute and short-lasting episode of anxiety, which often resembles a state of panic. When taken in high doses these effects can be intense and turn into attacks of panic and phobia. About 20–30% of users show brief acute anxiety reactions after smoking the drug. These symptoms tend to occur more in drug-naive subjects and in novel or stressful environments. In contrast, long-term users typically report that cannabis reduces their anxiety (Crippa et al, 2009).

Anxiety is also one of the main manifestations of the cannabis withdrawal syndrome. Its onset is typically between the second and sixth day and lasts from 4 to 14 days.

The main active psychoactive ingredient in cannabis, delta-9-tetrahydrocannabinol (THC), affects serotonergic, noradrenergic, GABAergic and glutamatergic transmission, but there is relatively little known about how it exactly induces anxiety. One of the reasons being that the psychotic effects of cannabis consumption has received way more attention, likely because it is a more severe side effect than the anxiety symptoms. Cannabidiol has been associated with an anxiolytic effect, perhaps through its effects on 5HT1A receptors (Crippa et al, 2009).

It has been shown that the amount of anxiety also depends on the genetic make-up of the user (Otten et al., 2016).

Acute intoxication could also lead to anxiety secondary to impaired cognitive functioning and the clouding of consciousness (Crippa et al, 2009), as mentioned in the question by the OP.

References
- Crippa et al., Hum Psychopharmacol Clin Exp (2009); 24: 515–23
- Otten et al., Addict Biol (2016); [Epub ahead of print]