There are valid safety concerns for cannabis use.

Medical cannabis should be offered only when primary treatments are not adequate in controlling the patient’s symptoms.

Psychoactive Effects:

Possible psychoactive effects include euphoria, relaxation, enhanced sensory experiences, altered time perception, agitation, loss of inhibition, amnesia, elevated self-importance, distrust, fear projection, and others.

Negative Side Effects of Cannabis Use:

Studies of potential side effects have identified nausea, fatigue, weakness, mood change, suicidal thoughts, hallucinations, dizziness, fainting and feelings of intoxication. There have been reports of seizures. Some studies show the risk of serious physiological effects to be about 1%.

Cannabis and Addiction:

An Institute of Medicine study found addiction rates for cannabis at 9%, lower than:
Tobacco: 32%
Heroin: 23%
Cocaine: 16%
Alcohol: 15%
Anti-anxiety drugs: 9%

Stopping Cannabis Use:

People who stop using medical cannabis may experience cannabis withdrawal syndrome.

Symptoms may include mild-irritability, anxiety, insomnia, disturbing dreams, suppressed appetite, restlessness, depressed mood. Onset is generally 24 to 72 hours after last use and usually peaks within the first week. The duration is usually 1 to 2 weeks, however sleep difficulties may last more than 30 days.