How Far Back Does a Hair Strand Drug Test Go?

Hair strand drug testing is frequently relied upon in family law and social services proceedings, yet it is also one of the most misunderstood forms of forensic testing. One of the most common questions asked by solicitors, local authorities and individuals involved in proceedings is how far back a hair strand drug test can show drug use.

The answer is not as straightforward as many people expect. Hair strand testing can provide a historical picture of drug use over several months, but the timeframe is always approximate rather than exact. Results depend on hair length and type, growth rates, analytical method used and, crucially, expert interpretation.

In cases involving children and family, decisions should never be based on Society of Hair Testing (SoHT) cut-off levels alone. In these cases, hair strand testing should be interpreted as expert opinion evidence, with a clear understanding of what it can and can’t tell us.

How hair strand drug testing works

Hair strand drug or alcohol testing is designed to detect substances and their metabolites that have become incorporated into growing hair fibres via the bloodstream following ingestion. As hair grows, these substances become incorporated into the hair shaft. They remain in the hair as it continues to grow, providing a historical record of substance use.

Scalp hair grows at an average rate of approximately 1.2 centimetres per month, although some hair may grow at a slower rate. This growth rate allows laboratories to use hair as a timeline, with sections of hair broadly corresponding to different periods of time. However, it is important to recognise that hair growth varies naturally between individuals and even across different areas of the scalp.

There is also a short delay between drug use and when it becomes detectable in a hair sample as it takes approximately 2 weeks for a growing hair follicle to penetrate the scalp. This means hair strand testing is not suitable for identifying very recent drug use in the hours or days immediately prior to testing. It is instead designed to establish evidence of historical use.

When instructing a hair strand drug test or a drug test using hair, it is therefore essential to understand that results reflect past exposure rather than immediate intoxication.

How far back can a hair strand drug test detect drug use?

In most cases, hair strand testing can look back up to 12 months, provided there is sufficient hair length available for analysis. The practical limit is most often determined by the length of the hair sample collected rather than the inherent detectability of the drug within the hair matrix.

As a general guide, approximately 1.2 centimetre of hair represents around 1 month of growth. Using this principle:

• 3.6 centimetres of hair represents roughly 3 months
• 7.2 centimetres represents roughly 6 months
• 14.4 centimetres represents roughly 12 months

Testing cannot evidence drug use beyond the available hair sample. If an individual has short hair or has had their hair cut recently, the timeframe that can be assessed will be shorter.

It is also important to note that these timeframes are approximate. Hair does not grow at an identical rate in every person, and variations are entirely normal. Expert interpretation is required to place results within an appropriate temporal context.

Segmental Analysis vs Overview Analysis

Hair strand testing can be carried out in different ways depending on the purpose of the instruction and the level of detail required.

Segmental analysis

Segmental analysis involves dividing the hair sample into consecutive 1.2 centimetre sections, with each section representing approximately 1 month of hair growth. This approach allows month by month assessment of drug use patterns.

Segmental testing can help identify whether drug use has been increasing, decreasing or ceased altogether during the period examined. It is particularly useful in family court cases where questions of abstinence, relapse or sustained change are central to informed decision making.

When a court approved hair strand test is required, segmental analysis often provides the most meaningful evidence, as it allows patterns to be explored rather than relying on a single averaged result.

Overview analysis

Overview analysis combines longer sections of hair, commonly 3 months at a time, to produce an averaged result across that period. While this approach can be useful to provide a general picture of an individual’s drug use, it is less informative.

Overview testing cannot narrow down when drug use occurred within the timeframe analysed. A positive result indicates use at some point during the period, but not whether that use was recent, historical, sporadic or repeated.

For this reason, overview analysis should be used cautiously in family proceedings and should not be treated as definitive evidence of behaviour at a specific point in time.

Factors that influence hair strand test results

Hair strand testing is influenced by several biological and external factors. A robust interpretation must consider all relevant variables to avoid unfair or misleading conclusions.

As we have outlined elsewhere, hair colour and melanin content can affect drug incorporation, as some substances bind more readily to darker hair. Natural variations in hair growth rate also mean that timelines cannot be treated as exact.

Cosmetic treatments such as bleaching, dyeing or chemical straightening may reduce drug concentrations in hair, although they do not reliably remove all evidence of drug use. Environmental or passive exposure is another key consideration, particularly where individuals may have been in contact with drugs without ingestion, for example via sweat if they share a bed with a regular substance user.

Declared prescribed medications must also always be reviewed, as some medications may legitimately account for detected substances or metabolites.

At Forensic Testing Service (FTS), all relevant factors are considered as part of the interpretative process to ensure our reports are legally defensible, based on all the facts, and scientifically sound. The courts have repeatedly stated that drug testing providers must fully and faithfully explain all findings, a position that FTS fully respects and supports. When the future of a family is being considered, forensic toxicology data must be interpreted as accurately and with as much context and explanation as possible.

Can hair strand testing detect alcohol use?

Hair testing is not limited to drugs. It can also be used to assess alcohol consumption through the detection of specific biomarkers associated with alcohol intake.

Hair strand testing for alcohol is commonly used in family court and social services cases where alcohol use is a concern. Unlike drug testing, alcohol testing does not measure the ingested drug directly, in this case ethanol, but instead analyses for specific biomarkers that reflect repeated alcohol exposure over time.

Testing hair for alcohol follows different analytical principles and requires specialist interpretation. Results can help distinguish between abstinence, low level consumption and excessive or chronic drinking patterns, but they must always be considered alongside other evidence.

Results from hair strand testing for alcohol should never be viewed as a simple pass or fail outcome. The individual’s declared consumption and case history remain essential.

Why expert interpretation is essential

Hair strand drug testing is a powerful evidential tool, but only when used appropriately. UK law is clear that results should never be presented in isolation or treated as definitive proof of behaviour.
Expert interpretation considers the limitations of testing, the analytical method used, individual biological factors and information provided through questionnaires and case documentation. Without this context, there is a real risk of misinterpretation.

In family law and social services cases, where decisions may have lifelong consequences for children and families, reliance on simple yes/no results or certificates of analysis is not appropriate. Hair strand drug test cases where social care teams are involved require carefully considered expert opinion evidence of all findings to ensure fairness and accuracy.

FTS does not provide results based on Society of Hair Testing (SoHT) cut-off levels, which have been shown to distort and omit relevant findings. Our role is to support instructing professionals by providing clear, balanced expert opinion evidence and interpretation that assists the court. We do so by reporting all findings and explaining their significance, as well as explaining what the results can and cannot show.

Frequently Asked Questions (FAQs):

How many months of drug use can a hair strand test show?
Typically up to 12 months, depending on the length of hair available for testing.

Can a hair strand drug test show recent drug use?
Yes, hair strand testing can be used to understand historical alcohol use. This is done through the analysis of alcohol biomarkers. The key alcohol biomarkers FTS test for in hair are Ethyl glucuronide (EtG) and Ethyl Palmitate (EtPa).

We can test for LFT, CDT, GGT-CDTr, MCV and PEth through blood tests.

We can also test for EtG via urine and nail tests. The detailed and complementary information provided from using a combination of blood and hair testing allows for more informed interpretations to be made, enabling clear and meaningful recommendations, so FTS would recommend a mix of these.

Yes, hair strand testing can detect alcohol consumption through biomarkers such as Ethyl glucuronide (EtG) and Ethyl Palmitate (EtPa), typically covering a 3-6 month period. For information on combining hair testing with blood, urine, and nail tests for alcohol, read our guide on testing alcohol use over time.

Does hair strand testing show occasional or one-off drug use?
Occasional or low-level use may not always be detected. Detection depends on the substance, frequency of use and analytical sensitivity.

Can hair strand testing detect alcohol consumption?
Yes, hair strand testing can be used to understand historical alcohol use. This is done through the analysis of alcohol biomarkers. The key alcohol biomarkers FTS test for in hair are Ethyl glucuronide (EtG) and Ethyl Palmitate (EtPa).

We can test for LFT, CDT, GGT-CDTr, MCV and PEth through blood tests.

We can also test for EtG via urine and nail tests. The detailed and complementary information provided from using a combination of blood and hair testing allows for more informed interpretations to be made, enabling clear and meaningful recommendations, so FTS would recommend a mix of these.

Is hair strand testing accepted by family courts and local authorities?
Yes, when carried out by an appropriate accredited laboratory and accompanied by expert interpretation.

What happens if there is not enough head hair available for testing?
Alternative sampling may be considered, such as body hair or nails.

Can hair colour or hair treatments affect the results?
They can indeed influence concentrations and therefore must be considered during interpretation.

Why should hair strand test results be interpreted by an expert?
Because results are complex and context specific, and misinterpretation can lead to unfair or incorrect conclusions.

Do you need to order a hair strand test?
If you require guidance on whether hair strand testing is appropriate for your case or would like to discuss the most suitable testing approach, our experienced case managers are available to provide advice.

You can request a quote, make an instruction or speak to a member of the Forensic Testing Service (FTS) team. Contact us by email: expert@forensic-testing.co.uk or call us on +44 (0)1924 480272.