What happens after a person smokes marijuana?

Within a few minutes of inhaling marijuana smoke, the user will likely feel, along with intoxication, a dry mouth, rapid heartbeat, some loss of coordination and poor sense of balance, and slower reaction time. Blood vessels in the eye expand, so the user's eyes look red.
For some people, marijuana raises blood pressure slightly and can double the normal heart rate. This effect can be greater when other drugs are mixed with marijuana; but users do not always know when that happens.
As the immediate effects fade, usually after 2 to 3 hours, the user may become sleepy.

How long does marijuana stay in the user's body?

THC in marijuana is readily absorbed by fatty tissues in various organs. Generally, traces (metabolites) of THC can be detected by standard urine testing methods several days after a smoking session. However, in heavy, chronic users, traces can sometimes be detected for weeks after they have stopped using marijuana.

Can a user have a bad reaction?

Yes. Some users, especially someone new to the drug or in a strange setting, may suffer acute anxiety and have paranoid thoughts. This is more likely to happen with high doses of THC. These scary feelings will fade as the drug's effects wear off.
In rare cases, a user who has taken a very high dose of the drug can have severe psychotic symptoms and need emergency medical treatment.
Other kinds of bad reactions can occur when marijuana is mixed with other drugs, such as PCP or cocaine.