Short-Term Effects

Using cannabis will probably make you feel more relaxed, free and open. Colours will seem brighter, sounds and smells more distinct. Some users feel happy and start talking a lot; others get quiet and withdrawn. Minutes can seem like hours, and ordinary objects seem to have special meaning.
If you smoke cannabis, you will probably feel the “high” quickly, and it will last two to four hours. If you eat it, the high happens later, and you feel it for a longer time.
Cannabis makes you clumsier and slow to react. Driving and operating machinery while stoned is not safe, especially if you combine cannabis with other drugs, including alcohol.
While high on cannabis, you lose some of your ability to learn. You can forget things, and have trouble concentrating—a serious problem for students.
Some users feel severe anxiety and high doses can cause panic attacks, fearful suspicious feelings (paranoia) and temporary psychosis. These effects usually disappear within hours.
After very high doses, you might hallucinate, but this is unusual.

Effects of Long-Term Use
Using cannabis heavily for a long time can have serious side effects:

Cannabis smoke contains cancer-producing chemicals. Smoking cannabis damages the lungs and can lead to chronic coughing and lung infections. People who smoke both marijuana and tobacco may develop lung, neck and head cancers at a younger age than those who smoke only tobacco.
Many people who use cannabis heavily for a long time have problems with short-term memory, concentration and abstract thinking. Most of these problems disappear after a few weeks without drugs, but some last for years.
Some heavy cannabis users appear less active and ambitious than other people. We cannot say that cannabis causes this. However, frequent use can make people even less motivated.