How do drugs and alcohol enter hair – and how long do they stay there?

Hair testing for drugs is a complex, specialist, lab-based process, but the questions our team are asked on a day-by-day basis are firmly rooted in the real world. They reflect hair testing’s growing contribution to social work, family law and workplace drug testing.

Two questions crop up more often than most. How do drugs and alcohol get into hair – and how long do they stay detectable with hair strand testing?

Here’s what you need to know.

How do drugs and alcohol enter hair?

Substances can enter hair in two ways.

First, internally. When an individual ingests alcohol, or inhales or injects drugs, the body breaks down these substances into smaller substances called metabolites. These metabolites, together with parent drug not metabolised, enter and circulate in the bloodstream. From there, the metabolites are incorporated into the hair cells on the scalp and body, and into the strands of hair themselves.

Alternatively, a donor can be exposed to drugs externally, by sharing an environment with another using the substance – for example, a bed or a room. In this case, traces of the substance will be found on the outside of the hair fibre.

In both instances, traces of drugs will be present in the hair sample when it is received at the test laboratory. Lab teams then use chemicals to disintegrate the hair sample ready for testing – at which point it can become difficult to distinguish between internal and external contamination (if the sample is not decontaminated before.)

For this reason, the party collecting the hair sample plays a key role in making sure the results of a hair drug test are interpreted accurately. It is their responsibility to provide context to the laboratory. We can give a highly accurate picture of whether drugs are in the person’s system or not, but without knowing their previous drug history or recent activity which could affect the results of the test.

With this knowledge, the laboratory will be able to interpret their test results correctly – and avoid making an error which could lead to an incorrect court judgement. It’s also the responsibility of the laboratory, prior to disintegrating the hair sample, to wash the intact hair sample to remove impurities or any eventual external contamination.

How long are drugs detectable in hair?

Most drugs are detectable in hair samples for as long as the hair sample is undamaged. The window for detection is long - hair tests have even been successfully used on Ancient Egyptian mummies to build a picture of their daily lives. What matters is not the age of the hair sample, but its length.

Hair on the human scalp grows roughly 1cm each month. A 1cm section of hair can, therefore, be tested for drug use in that month. 3cm of hair cut from the scalp will indicate substance use over three months, and so on.

It’s important to note that hair only appears above the scalp after around seven days. This means hair testing is not appropriate for detecting substance use in the week before substance collection. However, it is is a great indicator of the donor’s lifestyle with respect to drug. Moreover, drugs and metabolites may still be detectable in hair samples several months after a donor was last exposed to a substance, so retesting may be needed.

Donors can only remove traces of drugs or alcohol by cutting their hair – making hair tests an extremely accurate means of detecting substance use. Moreover, bleaching hair has a limited effect on reducing the number of detectable metabolites in a sample. Shampooing has no discernable effect.