The effects of cannabis are caused by the chemical compounds in the plant, including cannabinoids, such as tetrahydrocannabinol (THC), which is only one of more than 100 different cannabinoids present in the plant. Cannabis has 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 include euphoria and anxiety. Cannabidiol (CBD), another cannabinoid found in cannabis in varying amounts, has been shown to alleviate the adverse effects of THC that some consumers experience. With very high doses, THC can induce auditory and visual hallucinations.

Research about medical benefits of cannabis has been hindered by United States federal law. Smoking any substance will carry the same risk as smoking tobacco due to carcinogens in all smoke and the ultimate conclusions on these factors are disputed.