-different types of drugs have the potential to bind to your hair in more ways than one, and subsequently make chemical services more challenging. 

-the first way that drugs can enter your hair is by ingesting them. Your hair follicles and the sebaceous glands around them are fed by your circulation system, so that is how it enters. 

-once inside the hair, weak base drugs will bind to the melanin because it is acidic. Therefore the more melanin present (aka the darker the hair) the more drugs will be present in the hair.

-nicotine, cocaine, and morphine are common examples of weak base drugs. 

-anything that destroys melanin will decrease the binding of drugs.

-past drug use will appear down the hair strand in measurements proportional to the number of months since ingestion.

-the drugs remain in the hair for 2-8 months, even after just one dose. They can be detected as soon as 8 hours after drug use.

-the second way that drugs can enter hair is by enviroment. Hair that is exposed to the air while one is smoking nicotine, heroin, opium, or cocaine will be affected. In this case the binding pattern is not limited to regrowth; therefore a timeline cannot be established with certainty.

-there is a way to determine exactly how drugs entered the hair for when it matters. This is by testing for metabolites, which are converted from drugs by the liver. If these metabolites are found in the hair, it is known with certainty that the drugs didn't enter by enviroment. But that's really only relevant in a courtroom. As far as salons go, the consequences are the least to my limited knowledge.

-How might drugs affect the hair? They would decrease the overall effectiveness of whatever chemical process you are attempting, and/or make your hair "go brassy." 

...just another reason to not do drugs, kids! And stay in school!