In a study reported in the American Journal of Psychiatry, December 2001, marijuana abuse was said to increase the odds that a person would suffer from depression by four times. Other studies examined in the British Medical Journal, November 2002, reported similar findings. There are several reasons why marijuana gives rise to depression according to these studies.
The chemical composition is such that marijuana is considered to be a depressant. By its nature, marijuana reacts with chemicals in the body in the same manner as other depressive substances.
The chronic use of marijuana renders a person more socially withdrawn and apathetic which can result in depression in the long term. This is known as amotivational syndrome.
Marijuana may lead to depression is what might best be called the masking effect. A regular user of marijuana will not directly resolve different problems or issues in her life, resulting in depression.
According to the Mayo Clinic, the link between depression and marijuana depends upon a person being a regular or heavy user of the drug.
Further Research Required
The Mayo Clinic also reports that while there are studies supporting the proposition that marijuana increases the incidence of depression, more research is necessary to establish a definitive link.