Cannabis
According to current studies, cannabis use is associated with increased risk of psychotic disorders, and the more often cannabis is used the more likely a person is to develop a psychotic illness. Furthermore, people with a history of cannabis use develop psychotic symptoms earlier than those who have never used cannabis. Some debate exists regarding the causal relationship between cannabis use and psychosis with some studies suggesting that cannabis use hastens the onset of psychosis primarily in those with pre-existing vulnerability. Indeed, cannabis use plays an important role in the development of psychosis in vulnerable individuals, and cannabis use in adolescence should be discouraged. Some studies indicate that the effects of two active compounds in cannabis, tetrahydrocannabinol (THC) and cannabidiol (CBD), have opposite effects with respect to psychosis. While THC can induce psychotic symptoms in healthy individuals, limited evidence suggests that CBD may have antipsychotic effects. Methamphetamine
Methamphetamine induces a psychosis in 26–46 percent of heavy users. Some of these people develop a long-lasting psychosis that can persist for longer than six months. Those who have had a short-lived psychosis from methamphetamine can have a relapse of the methamphetamine psychosis years later after a stressful event such as severe insomnia or a period of hazardous alcohol use despite not relapsing back to methamphetamine. Individuals who have a long history of methamphetamine use and who have experienced psychosis in the past from methamphetamine use are highly likely to re-experience methamphetamine psychosis if drug use is recommenced.
Individuals who experience substance-induced psychosis tend to have a greater awareness of their psychosis and tend to have higher levels of suicidal thinking compared to those who have a primary psychotic illness. Drugs commonly alleged to induce psychotic symptoms include alcohol, cannabis, cocaine, amphetamines, cathinones, psychedelic drugs (such as LSD and psilocybin), κ-opioid receptor agonists (such as enadoline and salvinorin A) and NMDA receptor antagonists (such as phencyclidine and ketamine). Caffeine may worsen symptoms in those with schizophrenia and cause psychosis at very high doses in people without the condition. Cannabis and other illicit recreational drugs are often associated with psychosis in adolescents and cannabis use before 15 years old may increase the risk of psychosis in adulthood. Alcohol
Approximately three percent of people with alcoholism experience psychosis during acute intoxication or withdrawal. Alcohol related psychosis may manifest itself through a kindling mechanism. The mechanism of alcohol-related psychosis is due to the long-term effects of alcohol consumption resulting in distortions to neuronal membranes, gene expression, as well as thiamin deficiency. It is possible that hazardous alcohol use via a kindling mechanism can cause the development of a chronic substance-induced psychotic disorder, i.e. schizophrenia. The effects of an alcohol-related psychosis include an increased risk of depression and suicide as well as causing psychosocial impairments. Delirium Tremens, a symptom of chronic alcoholism which can appear in the acute withdraw phase, shares many symptoms with alcohol-related psychosis suggesting a common mechanism.