Marijuana is not physically addictive, despite what many anti-marijuana people want us to believe. Marijuana users can use it regularly, even multiple times daily, without any problem giving it up.
A very small minority of people (heavy chronic users) who use marijuana can find that they have developed a psychological addiction. This can be as mild as some difficulty sleeping, slight irritability, and moodiness. Since most people who use marijuana are not heavy chronic users, this is not common at all. If you are concerned about any possibility of psychological addiction it is very easily avoided by taking time off from regular use. For instance, if you use it every day, you should take one day off per week or take one week off every three months. Even without taking this time off from use, the majority of people using marijuana report that they have no difficulty stopping usage.
Marijuana has the lowest risk of dependence and withdrawal potential if you compare it to other substances, such as alcohol, opiates (like Vicodin), caffeine, and other psychoactive drugs. Many of the anti-depressants (currently being prescribed abundantly) have very severe physical withdrawal symptoms which most patients are not warned about.
People who are addiction-prone can have trouble controlling their use of marijuana. This is an issue for about 10% of users (this number comes from looking at recreational users, not those using marijuana for medicinal purposes). A person who is considered a marijuana abuser is one who uses it to excess and finds that the use interferes with normal every day activities, like maintaining personal hygiene, going to work, having normal interpersonal relationships. Persons who find that they cannot control their use may find abstinence is preferred.
Most medical marijuana patients find that they cannot function on the medications that they have been prescribed for pain, insomnia, depression, etc. and find that they function very well on medical marijuana. It can be said that this ability to maintain a normal life without dependence and addiction is what is driving many patients to choose medical marijuana as their preferred medication.
Additionally, another compelling reason that most medical marijuana patients have switched from opiates (like Vicodin), antidepressants, or sleeping pills is exactly for the reason that marijuana is not physically addictive and can be easily stopped.