Substance abuse is a worldwide phenomenon. It is on the increase in most countries and claims the lives of millions every year. Substance abuse may involve licit and illicit drugs, with licit substances claiming more lives than illicit drugs. Illicit substance abuse is on the increase, especially with new drugs emerging on the world market every year. These new drugs appear faster than scientific studies can keep pace in determining their possible detrimental influences on health. Many abused drugs do have oral health complications. For this reason, it is important for dentists to have a thorough knowledge of the oral environment to be able to detect any abnormalities, regardless of what the underlying cause may be. Due to the nature of illicit substance abuse, reliable information and science is hard to come by. This overview will focus on the direct consequences for oral health, whilst acknowledging that substance abuse may also have direct and indirect influences on general health.