Hair drug testing explained

Hair drug testing is considered the gold standard in toxicological analyses. This long-established method can provide a history of drug abuse and is routinely used in the forensic and family law fields. It is the norm for samples of hair to be collected during criminal investigations (drug-related deaths, drug-facilitated crime, child protection, etc.) and for monitoring drug misuse (such as in workplace drug testing and driving licence re-granting following drug-driving convictions).

Oral fluid and urine testing can only give a short-term measure of drug use (usually a few days or weeks). The advantage of hair is that it is a strong, stable tissue, is less affected by contaminants and requires no special conditions for storage, such as refrigeration. Drugs remain relatively stable in hair and can thus be stored indefinitely. As such, hair strand testing can effectively confirm long-term exposure to drugs over a defined period depending on the length of hair collected. Known as a ‘wide window’ form of testing, it can determine a history of drug intake for up to 12 months.

After ingesting a drug, it is absorbed into the bloodstream. Drugs and their metabolites are then absorbed through the root of the hair and become embedded in the hair shaft. These metabolites remain in the hair as it grows, hence it is a suitable method for detecting a person’s historical drug intake on a month-by-month basis. Unlike saliva and urine tests, hair strand analysis cannot show recent drug history: it can take between 7–10 days for the hair containing the drug to grow above the scalp. Oral fluid testing is therefore preferred for post-incident testing as it can detect drugs in saliva approximately 30–60 minutes after ingestion.

How is a sample taken?

The accuracy of any hair drug analysis depends on both the sampling procedure and the laboratory techniques employed. Hair strand samples are normally collected by a qualified nurse or trained sample collector.

The hair sample needs to be cut from the highest point of the scalp (the vertex) as this region is associated with least variation in growth rates. Ideally, the sample needs to contain around 200 individual strands (about the width of a pencil) and should always be taken from a discrete area.

Hair samples are usually taken from a person’s head; however, if head hair is not available alternative collection sites could be considered including pubic, underarm, chest, leg and beard hair. The growth rate of hair from these alternate sites differs from head hair and as such body hair cannot be used to determine a specific timeframe of drug use.

If an individual has been in an environment heavily laden with a drug, detectable levels could be found in the hair sample due to smoke. In addition, it could be transferred by direct contamination (such as by hands). To avoid any such false positive results, each hair sample is chemically washed three times to remove or reduce any drug present prior to analysis. In addition, the Society of Hair Testing (SoHT) Guidelines for Drug Testing in Hair provide recommended cut-off levels to follow [1]. The washing solution can also be analysed if required. For example, if an individual admits to being surrounded by cannabis smokers, the washing solution could be used to back up the test results. Any external exposure to drugs should be declared at the time of sample collection.

Excessive shampooing, some cosmetic treatments (such as dyeing, bleaching, perming and relaxing), and the use of thermal straighteners may reduce drug concentrations in hair to varying degrees. The extent of the loss will depend on the cosmetic treatment used and the drug present. All hair treatments should therefore be declared at the time of sample collection and taken into consideration when reviewing the findings of a hair test.

Types of tests

There are two types of hair drug tests available: overview or segmented analysis. Head hair grows at an average rate of 1 cm each month; therefore, drug use can be established over a defined period depending on the length of hair selected.

An overview analysis is beneficial to obtain a general indication of drug use. This could range from 3 cm to identify drug use within that 3-month time period, up to 6 or even 12 months, dependent on the length of hair available. A 6-month overview could also be used to provide two results: one for the first 3 months and a second result for the later 3 months. If a more detailed analysis is needed, segmented would be the preferred option.

By segmenting head hair samples into monthly 1 cm sections, a month-by-month profile of drug use can be attained to provide a detailed historical profile of an individual’s drug exposure. In cases of drug-related crime, where a single administration of a particular drug is suspected, the analysis of small segments can help establish a positive hair test in a narrow timeframe.

If a hair drug test was to cover 6 months, a month-by-month analysis will provide six individual results for each of the months. As such, segmented analysis is especially useful if you need to obtain a trend in drug use, such as to see a decrease or increase in drug use over time, or to highlight intermittent use.

In an overview test, any episodes of drug taking are averaged out over the period being tested. For example, if an individual has not taken drugs for 5 months but then takes an excessive amount in 1 month, drug markers will be found along the entire length of the hair. In the case of segmentation, the episode of drug taking can be pinpointed to the actual month.

Legal implications

Deciding on which type of hair drug testing to choose in legal cases can have major implications. If a 3-month overview drugs of abuse test is selected, even if an individual uses drugs occasionally, the test may come back positive for the whole 3-month period. This result implies that the sample donor had been abusing drugs during the whole period analysed.

To overcome this, segmented hair drug testing could be performed over the same 3-month period. The sample donor could show positive one month and negative the other 2 months. This would provide a more accurate assessment of the individual’s drug abuse/abstinence when presented to the court for scrutiny.

Cases such as this demonstrate the importance of an expert advising on the appropriate test to use. Taking an average of drug levels in a specified hair section without the knowledge of levels in the hair section prior to the period of interest can produce misleading results.

The table below is an example of the hair analyses regularly performed by AlphaBiolabs.

Analysis type required
Number of segmentsReported time periodType of test
313 monthsOverview
333 individual monthsSegmented
622 x 3-month periodsOverview
666 individual monthsSegmented



For expert advice on hair testing and other drug testing solutions, please call 0333 600 1300 or email us at