Heavy smokers of cannabis develop a tolerance to the subjective and cardiovascular effects of cannabis. Some users report a withdrawal syndrome on cessation of use with symptoms that may include restlessness, irritability, mild agitation, insomnia, sleep disturbance, nausea and cramping. Controlled laboratory studies have observed withdrawal symptoms, which were short-lived and abated after four days. They do not last long if they happen! However, there are still methodological problems to be addressed in measuring the severity of these withdrawal symptoms. There is some evidence to suggest that a cannabis dependence syndrome occurs with heavy cannabis use marked by difficulty in controlling use and continued use despite experiencing adverse personal consequences. American studies have found that about one in ten of those who ever use cannabis become dependent on it during the four or five years of heaviest use. However, the risk of dependence is more like that for alcohol, than tobacco and opioids. This may be due to differences in drug effects, its availability or the penalties associated with the use of the drug or some combination of these.
