Many people are interested in using marijuana for therapeutic purposes as legalization expands across the country (USA). The impact of marijuana depends upon the consumed potency and dose of its major psychoactive components, tetrahydrocannabinol and cannabidiol, which have very different psychiatric effects. This Open Forum discusses what is known about the impact of these marijuana constituents on people with serious mental illnesses, who may experience many adverse effects and more easily develop addiction with marijuana use. As use of medical marijuana increases, mental health clinicians can help prevent or address adverse effects by informing themselves and others about marijuana’s effects and by coordinating with marijuana prescribers.. Login at top right hand side of page using your MPFT NHS OpenAthens for full text.