Background
About 25% of the worldwide population suffers from mental, neurological and substance use disorders but unfortunately, up to 75% of affected persons do not have access to the treatment they need. Data on the magnitude of the mental health problem in Kenya is scarce.

The objectives of this study were to establish the prevalence and the socio-demographic factors associated with mental and substance use disorders in Kosirai division, Nandi County, Western Kenya.

Methods
This was a cross sectional descriptive study in which participants were selected by simple random sampling. The sampling frame was obtained from a data base of the population in the study area developed during door-to-door testing and counseling exercises for HIV/AIDS. Four hundred and twenty consenting adults were interviewed by psychologists using the Mini International Neuropsychiatric Interview Version 7 for Diagnostic and Statistical Manual 5th Edition and a researcher-designed social demographic questionnaire.

Results
One hundred and ninety one (45%) of the participants had a lifetime diagnosis of at least one of the mental disorders. Of these, 66 (15.7%) had anxiety disorder, 53 (12.3%) had major depressive disorder; 49 (11.7%) had alcohol and substance use disorder. 32 (7.6%) had experienced a psychotic episode and 69 (16.4%) had a life-time suicidal attempt. Only 7 (1.7%) had ever been diagnosed with a mental illness. Having a mental condition was associated with age less than 60 years and having a medical condition.