If my 18 year old son lives with me and gets into trouble with the law for drugs or alcohol, am I responsible for his actions as being his parent?

No, your eighteen year old son is considered an "Adult" for purposes of criminal charges.  He alone would bear criminal responsibility for his actions unless you provided him with "Alcohol" or "Drugs".       For more information on Criminal Law, visit my website at: http://davidleelegal.com/ David Lee