Overview: It is well known now that nicotine is not the main reason why tobacco products are addictive. As for alcohol, several genes have now been identified for alcohol sensitivity. Tobacco and alcohol are the most widely used drugs in the world. It is well known now that nicotine is not the main reason why tobacco products are addictive. Tobacco smoke contains over four thousand other addictive chemicals. As for alcohol, scientists have nicknamed it the "dirty drug". It creeps into dozens of circuits of the brain and causes considerable damage. Several genes have now been identified for alcohol sensitivity.