The Alcohol, Smoking and Substance Involvement Screening Test (ASSIST) is the first screening test to cover all psychoactive substances including alcohol, tobacco and illicit drugs. It has been shown to be reliable, feasible, comprehensive and cross-culturally relevant in primary health care (PHC) settings in a number of internationally demonstrated studies. The present study aimed to describe the characteristics of patients in PHC settings in Thailand with regards to their substance use behaviours and responses to the ASSIST. Methods: All consecutive patients aged 16 to 65 years who visited a study hospital at the time of data collection were approached. Results: Of 775 patients, 747 were recruited into the study and the ASSIST was administered to them by trained research assistants and PHC workers. Among these, 7.1%, 67.9% and 25.0% were screened as high-, moderate- and low-risk levels for any substance use, respectively. Tobacco was the most common substance used followed by alcohol, marijuana, krathom leaves, amphetamine and krathom cocktail. Two hundred and forty five (245) moderaterisk substance users, excluding smokers, were assessed for their substance use behaviours, their readiness to change, their problems related to substance use, and their quality of life. The younger, middle and older age groups were statistically different in terms of substance use. Most patients were in the low and very low stages of change. Conclusion: Early detection and effective intervention is needed before substance users encounter substance-related problems. The ASSIST is suitable for use as a routine screening instrument and should be screened for teenagers and young adult patients who visit PHC facilities with particular emphasis on the popular substances of their age group.