The effects of cannabis are caused by chemical compounds in the cannabis plant, including 400 different cannabinoids such as tetrahydrocannabinol (THC), which allow its drug to have various psychological and physiological effects on the human body. Different plants of the genus Cannabis contain different and often unpredictable concentrations of THC and other cannabinoids and hundreds of other molecules that have a pharmacological effect, so that the final net effect cannot reliably be foreseen.
Acute effects while under the influence can sometimes include euphoria. Although some assert that cannabidiol (CBD), another cannabinoid found in cannabis in varying amounts, may alleviate the adverse effects of THC that some users experience, little is known about CBD effects on humans.
In the United States, medical cannabis research is limited by federal restrictions. Smoking any substance could possibly carry similar risks as smoking tobacco due to carcinogens in all smoke,  and the ultimate conclusions on these factors are disputed.
Cannabis use disorder is defined as a medical diagnosis in the fifth revision of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5).