Ageing society issues increases needs to better understand the relationship between cannabis, endocannabinoid system and mental disorders also for mental health professionals so that they can respond to increasing medical as well as recreational cannabis use among their patients. Knowledge about herbal cannabis, the endocannabinoid system, and cannabinoid pharmacology is rapidly expanding. However, compared with the literature on non-medical cannabis use, the scientific literature on therapeutic use of cannabis is underdeveloped, as noted in a recent systematic review of medical cannabis and mental health. Entourage effect combining about 400 molecules with different bioactivities is mediated in full complexity by herbal cannabis referred as plant material derived from the flowering tops of Cannabis sativa and indica biotypes. Regardless of the legal status of cannabis, many patients with psychiatric disorders use cannabis and report improvement in their symptoms. Patients use cannabis for symptoms of PTSD, anxiety disorders, depression, ADHD, bipolar disorder, chronic pain, insomnia, opiate dependence, and others. In addition, patients use cannabis for neurological conditions such as the spasticity of multiple sclerosis, agitation in dementia, neuropathic pain, and specific seizure disorders that are unresponsive to standard therapies. Patients also use cannabis to reduce the nausea and anorexia of cancer chemotherapies and to improve their mood and outlook frequently with their oncologist‟s approval. There is moderate evidence that cannabis or cannabinoids are effective for improving short-term sleep outcomes in individuals with sleep disturbances associated with a variety of conditions. While none of those conditions is regarded as a psychiatric disorder, they all can be associated with other psychiatric symptoms (beyond sleep disturbance) and are all encountered by psychiatrists as comorbid conditions in clinical practice: obstructive sleep apnea, fibromyalgia, chronic pain, and multiple sclerosis. Sleep disturbances are also ubiquitous in the nosological schemes of psychiatry, notably in PTSD where nightmares are often a factor.