Tetrahydrocannabinol

Tetrahydrocannabinol, or THC, is the chemical or drug made in the cannabis that causes a person to feel different. It is similar to at least 113 other chemicals in cannabis, known as cannabinoids, but not all of them will make you feel different or high after using it. There are about 540 chemicals in cannabis total. THC is a very fatty chemical and doesn't interact with water very well. THC is thought to be used by the plant for protection from bugs, light, and other harm from its environment. The THC chemical structure was discovered in 1964 by Dr. Raphael Mechoulam and Dr. Y. Gaoni at the Weizmann Institute in Israel.
Use in medicine.
THC is used in medicine as a treatment for many physical and psychological problems. It can be used to help people with pain or are sick from getting chemotherapy for cancer. There are some medications which have THC and can be ordered by a doctor depending on where and what country you live in.
Pharmacological Effects.
Cannabis works by sticking to special receptors in the brain and body, which is known as the endocannabinoid system or ECS. There are 2 kinds of receptors, CB1 and CB2. Over time, a person will need more THC to feel the same effects as before. This is known as tolerance. A person may also experience withdrawal, which can be uncomfortable and is a result of stopping THC use quickly after using it for some time. People usually do not get addicted to using THC.
THC in the body.
THC is very fatty (fat soluble) and binds to fat cells in our body and causes it to leave the body slowly, even if the effects of the drug or high has already ended. The amount still in the body depends on how often THC is used. The minimum time for a person to pass a drug test would be about 1 week. It is not possible for humans or animals to realistically die from taking too much THC or marijuana and no person has ever died from it.
Legality.
Many places in the world do not see THC or cannabis as legal and can have very harsh penalties from having, producing, or using it. Some countries allow it for use only in medical treatment while others allow it for recreational use by adults, usually older than 18 or 21 years old. Children may be permitted to use it if they're very sick.
In North America, Canada and Mexico have taken action to allow marijuana use in some way. In the United States, the federal government sees marijuana as illegal and very addictive and cannot be used as a medicine. Individual states with in the U.S.A. allows people to use marijuana to help with sickness or recreationally.
Drug Testing.
Employers or people who work in a government agency, like the police or FBI, may test someone to see if they used any drugs recently. This is called a drug test. Police may do this if they think you have used marijuana before or while you are driving a car. THC lasts a long time in the body and can be found in body fluids, like urine or saliva, or hair and sweat. Since it lasts so long in the body, it can be a problem being sure if someone is high.