Panic attacks may also be caused by substances. Discontinuation or marked reduction in the dose of a substance such as a drug (drug withdrawal), for example, an antidepressant (antidepressant discontinuation syndrome), can cause a panic attack. According to the Harvard Mental Health Letter, "the most commonly reported side effects of smoking marijuana are anxiety and panic attacks. Studies report that about 20% to 30% of recreational users experience such problems after smoking marijuana." Cigarette smoking is another substance that has been linked to panic attacks.