The effects on each person depend on the user's experience, as well as:
how strong the marijuana is (how much THC it has);
what the user expects to happen;
where (the place) the drug is used;
how it is taken; and
whether the user is drinking alcohol or using other drugs.

Some people feel nothing at all when they smoke marijuana. Others may feel relaxed or high. Sometimes it makes users feel thirsty and very hungry - an effect called "the munchies." Some users suffer bad effects from marijuana. They may suffer sudden feelings of anxiety and have paranoid thoughts. This is more likely to happen when a more potent variety of marijuana is used.

If someone is high on marijuana, he or she might
seem dizzy and have trouble walking;
seem silly and giggly for no reason;
have very red, bloodshot eyes; and
have a hard time remembering things that just happened.
When the early effects fade, over a few hours, the user can become very sleepy.

The short-term side effects of marijuana include:
problems with memory and learning;
distorted perception (sights, sounds, time, touch);
trouble with thinking and problem-solving;
loss of coordination; and
increased heart rate, anxiety.

These effects are even greater when other drugs are mixed with the marijuana; and users do not always know what drugs are given to them.

The long-term side effects from findings show that regular use of marijuana or THC may play a role in some kinds of cancer and in problems with the respiratory, and immune systems.

Cancer
It's hard to know for sure whether regular marijuana use causes cancer. But it is known that marijuana contains some of the same, and sometimes even more, of the cancer-causing chemicals found in tobacco smoke. Studies show that someone who smokes five joints per week may be taking in as many cancer-causing chemicals as someone who smokes a full pack of cigarettes every day.

Lungs and airways
People who smoke marijuana often develop the same kinds of breathing problems that cigarette smokers have: coughing and wheezing. They tend to have more chest colds than nonusers. They are also at greater risk of getting lung infections like pneumonia.

Immune system
Animal studies have found that THC can damage the cells and tissues in the body that help protect people from disease. When the immune cells are weakened, you are more likely to get sick.

Source: U.S. Department of Health and Human Services
NIH Publication Number 98-4037