The side effects of marijuana usage can vary and are dependent on a variety of factors. Tolerance to THC, the main psychoactive chemical found in marijuana, will usually determine what effects are felt and for how long. The mindset of the user, as with all psychoactive drugs, also plays an important part in deciding how the user will feel after consuming marijuana.

Side Effects in the Short-Term

People who use marijuana do so chiefly to experience a change in their perception, increase their relaxation, and to feel good in general. While consuming marijuana is usually reported to be a pleasureful activity by most users, harsh experiences known as "bad trips" can occur. While under the influence, a user's mind may seem more active, resulting in creative thoughts. Time distortion while high on marijuana may take place, leaving users feeling as though hours take place in the course of a few minutes. Dramatically increased enjoyment of everyday activities often takes place, making boring pastimes much more tolerable to the marijuana user. Short term memory is inhibited rather noticeably. Feelings of paranoia or of being in danger can occur with low doses, and at higher doses, particularly those consumed orally, users of the drug may go into panic attacks.

The physical effects experienced with marijuana can also vary widely. Although the main reason some people use the drug is for relaxation purposes, many other changes in behavior and appearance might take place. In some cases, relaxation can lead to lethargy and unwillingness to do much of anything. Marijuana is a powerful appetite stimulant, resulting in the use of the term "munchies". This has led to marijuana being prescribed to people who have trouble holding down their food, usually chemotherapy patients. Marijuana also numbs the sense of touch, and some stronger varieties of the cannabis indica species are used as painkillers both recreationally and as a prescription.

Obviously it is unhealthy to inhale smoke of any kind, marijuana smoke being no exception. Coughing, wheezing, and a variety of upper respiratory can result from consuming marijuana through smoking. Other common short term side effects associated with marijuana usage are reddening of the eyes, loss of coordination, and lightheadedness.

Marijuana Side Effects in the Long-Term

When a person has been using marijuana regularly over an extended period of time, many changes in mood and in some cases, mental health, may take place. Short and long-term memory are both decreased with repeated usage of the drug. As tolerance to the psychoactive chemical THC is increased, as is the amount of marijuana needed to afford the marijuana user an acceptable high. A user's mood may be affected negatively by this need to use more drugs to get high.

After marijuana use has been put to a stop, certain withdrawal symptoms may occur. Most common is boredom with everyday life while sober. Difficulty sleeping may also occur, as marijuana is used by some as a sleep aid. Some studies have shown that marijuana users are more likely to be mentally unhealthy, developing problems as extreme as psychosis, than people who do not use marijuana.