Hair follicle testing can detect traces of marijuana, cocaine, methamphetamines, opiates like morphine and codeine, and PCP. Some hair follicle tests also look for traces of alcohol, poisons and toxins in the hair, and tricyclic antidepressants.
Hair follicle tests typically show substances consumed within the three months prior to the test. This is because slightly over 1 inch of hair is cut off, which is about three months of normal hair growth. For the test to be positive, the lab assumes that the subject used the drug more than once within this hair growth period.
However, such testing is not effective in identifying drugs used within seven to 10 days before the test, because it can take about seven to 10 days for hair follicles with the substance to grow out from the scalp. If a drug was only used a few days before the test but not at all before, the test might come back negative.
Some drugs found in a hair follicle test might be a prescription or used for legitimate reasons, while others are clearly illicit drugs. For example, no doctors prescribe cocaine or PCP to their patients. However, if morphine appears in a subject's system, he might have the opportunity to present a note from his doctor stating it was prescribed. There are also some prescription pills with a small amount of methamphetamine that can cause a positive test result.