Cannabis, also known as marijuana, is a psychoactive drug from the Cannabis plant that is used for recreational or medical purposes. Cannabis is smoked or eaten. The predominant psychoactive substance in cannabis is tetrahydrocannabinol (THC). Cannabis THC use is very common in people with schizophrenia. It is important that clinicians know when their patient is using cannabis. When people with schizophrenia use cannabis THC, this makes their psychotic symptoms and cognition substantially worse. It is important to note that there is another substance in cannabis, cannabidiol (CBD), that does not cause psychotic symptoms. Cannabidiol is found in very low levels in most purchased cannabis, but can be obtained as a concentrate or by prescription. Cannabidiol may reduce psychotic symptoms in patients with schizophrenia when added to antipsychotic medication that they are taking.