The main active chemical in marijuana is THC (delta-9-tetrahydrocannabinol). It is a psychoactive ingredient. The highest concentrations of THC are found in the leaves and flowers. 
When marijuana smoke is inhaled, THC rapidly passes from the lungs into the bloodstream and is carried to the brain and other organs throughout the body. THC from the marijuana acts on specific receptors in the bra in, called cannabinoid receptors, starting off a chain of cellular reactions that finally lead to the euphoria, or “high” that users experience.
Certain areas in the brain, such as the hippocampus, the cerebellum, the basal ganglia and the cerebral cortex, have a higher concentration of cannabinoid receptors. These areas influence memory, concentration, pleasure, coordination, sensory and time perception. Therefore these functions are most adversely affected by marijuana use.
Marijuana’s strength is correlated to the amount of THC it contains and the effects on the user depend on the strength or potency of the THC. The THC content in marijuana has been increasing since the 1970s. For the year 2007, estimates from confiscated marijuana indicated that it contains almost 10 percent THC on average. There are many other chemicals found in marijuana, many of which may adversely affect health. Marijuana contains over 60 different cannabinoid compounds related to THC, including cannabidiol, cannabinol, and β-caryophyllene.
Marijuana is usually smoked as a cigarette (called a joint or a nail) or in a pipe or bong. In recent years, it has appeared in blunts, which are cigars that have been emptied of tobacco and refilled with marijuana, often in combination with another drug, such as crack. The “blunts” retain tobacco leaf used to wrap the cigar and therefore it combines marijuana’s active ingredients with nicotine and other harmful chemicals. Some users also mix marijuana into food or use it to brew tea.