How Far Back Does a Hair Strand Test Go?

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

The answer is not as straightforward as might be expected. 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 factors including the length and type of hair available, individual growth rates, the analytical method used and, crucially, expert interpretation.

This distinction is particularly important in proceedings involving children and families, where testing may contribute to decisions with significant consequences. Hair strand testing should be treated as expert opinion evidence, with a clear understanding of both what the results can indicate and their limitations. Decisions should not be based on Society of Hair Testing (SoHT) cut-off levels alone or on a simple positive or negative interpretation and court guidance supports this.

How hair strand testing works

Hair strand analysis is designed to detect substances and their metabolites that become incorporated into growing hair fibres following ingestion. As the hair grows, these substances are locked in the hair shaft, providing a historical record of drug exposure.

Scalp hair grows at an average rate of approximately 1.2 centimetres per month, although growth rates vary naturally between individuals and even across different areas of the scalp. This allows sections of hair to be used as an approximate timeline, with different lengths broadly corresponding to different periods.

There is also a delay between drug use and when it becomes detectable in a hair sample, as it takes approximately two weeks for growing hair to emerge through the scalp. Hair strand testing is therefore not suitable for establishing drug use in the hours or days immediately before collection. Nor should a particular section of hair be treated as representing an exact calendar period.

This is an important consideration when framing an instruction. Hair strand testing can provide evidence of historical exposure and patterns of drug use over time, but it cannot establish immediate intoxication or pinpoint the exact date on which a substance was consumed.

How far back can hair detect drug use?

In most court ordered cases, the requirement is for 3- or 6-months segmented analysis, provided there is sufficient hair length available. The practical timeframe is usually determined by the length of the hair sample collected rather than how long a drug remains detectable within the hair.

As a general guide, approximately 1.2 centimetres of hair represents around one 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

These periods should always be regarded as approximate rather than exact.

The available sample is therefore an important consideration when testing is being requested. If an individual has short hair, has recently had their hair cut or there is insufficient scalp hair, it may not be possible to assess the full period originally requested. Where this is identified at the collection stage, FTS case managers can discuss the available options with the instructing party, ensuring that expectations about the period that can realistically be assessed are clear before analysis begins.

Alternative samples, including body hair or nails, may be considered where appropriate, although they provide different information and should not simply be treated as direct substitutes for scalp hair.

Equally, a longer sample extends the period that may be assessed but does not provide more precise dates of drug use. The timeframe and sample available should therefore be considered together when deciding the most appropriate testing approach.

Segmental 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. The approach selected should reflect the question that needs to be answered.

Segmental analysis involves dividing the hair sample into consecutive 1.2-centimetre sections, each broadly representing one month of hair growth. This can help identify patterns over time, including whether drug use appears to have increased, decreased or ceased during the period examined.

This can be particularly valuable in family proceedings where questions of abstinence, relapse or sustained change are relevant. Rather than relying on a single averaged result, segmental analysis allows changes across the available timeframe to be considered. However, as hair growth rates vary, individual segments should not be treated as representing precise calendar months.

Overview analysis typically combines longer sections of hair, commonly three months at a time, producing an averaged result across that period. It can provide a general picture of drug use but cannot establish when within that period use occurred or whether it was sporadic or repeated.

The distinction is important when commissioning testing. If the evidential question concerns change over time, an overview result may not provide the level of detail required.

For example, an instruction seeking to establish whether there is evidence of drug use at any point during a six-month period is different from one asking whether reported abstinence has been maintained throughout that period. In the latter case, analysis across shorter segments may provide more useful information about changes or patterns over time. The testing strategy should therefore reflect the specific evidential question rather than simply the overall period under consideration.

Factors that influence hair strand test results

Hair strand testing is influenced by a range of biological and external factors. These need to be considered when interpreting results to avoid conclusions that may be misleading or place too much certainty on the analytical findings alone.

Hair colour and melanin content can affect drug incorporation, as some substances bind more readily to darker hair. Natural variations in hair growth also mean that timelines cannot be treated as exact. Cosmetic treatments, including bleaching, dyeing or chemical straightening, may reduce drug concentrations in hair, although they do not reliably remove all evidence of drug use. For example, the repeated use of modern hair straighteners can also impact how drugs and metabolites behave along the hair shaft due to the application of heat.

Different substances may also behave differently within the hair. For example, cannabis has particular characteristics that can affect its incorporation and distribution along the hair shaft, which should be taken into account when interpreting findings in relation to historic use.

Environmental or passive exposure must also be considered. An individual may come into contact with a substance without having ingested it, including through close contact with a regular substance user. Declared prescribed medications should also be reviewed, as these may legitimately account for substances or metabolites identified during analysis.

Importantly, an absence of a drug from a hair sample does not necessarily prove that no use occurred during the period examined. Occasional or low-level use may not always be detected, with detection influenced by factors including the substance involved, frequency of use and analytical sensitivity.

These factors demonstrate why laboratory findings need to be considered alongside the individual's circumstances and other available evidence. FTS considers relevant contextual information as part of the interpretative process, with findings and their significance explained as fully as possible.

Can hair strand testing detect alcohol use?

Hair can also be used to assess alcohol consumption, although the analytical approach differs from drug testing. Hair testing for alcohol analyses specific biomarkers associated with alcohol consumption over time, rather than measuring the ingested drug directly, in this case ethanol.

This can provide useful evidence in family law and safeguarding cases where alcohol use is a concern. However, results require specialist interpretation and should be considered alongside the individual’s declared consumption, case history and other available evidence.

As with drug testing, hair alcohol results should not be treated as a simple pass or fail outcome. Where a more complete picture of alcohol use is required, other testing methods may also be considered alongside hair analysis.

Framing the Instruction

The value of hair strand testing depends partly on the question it is being asked to answer. Before testing is instructed, it is important to identify the period of interest and whether the purpose is to understand general patterns of drug use, investigate a period of claimed abstinence or explore whether behaviour appears to have changed over time.

The available sample also needs to be considered. Requesting six or twelve months of analysis does not necessarily mean that period can be assessed if there is insufficient scalp hair. Equally, where change over time is important, an overview analysis covering several months may not provide the level of detail required.

Relevant background information should also accompany the instruction. Declared drug use, prescribed medication, hair treatments and other circumstances that may affect interpretation can all be important when the expert considers the analytical findings.

Discussing the evidential question with the testing provider before collection can help ensure that the sampling and analytical strategy are appropriate. This can avoid commissioning testing that is not suited to its intended purpose and help ensure the resulting report provides evidence that is useful within the proceedings.

Why expert interpretation is essential

Hair strand drug testing can provide valuable evidence, but only when the findings are interpreted appropriately. Results should not be considered in isolation or treated as definitive proof of an individual’s behaviour.

Expert interpretation considers the limitations of the testing, the analytical method used, individual biological factors and information provided through questionnaires and case documentation. It also considers the wider circumstances of the case and explains both the significance and limitations of the findings.

This is particularly important in family law and local authority proceedings, where hair strand testing may contribute to important decisions about a child or family. A simple yes/no result or Certificate of Analysis does not provide the contextual interpretation required to understand the evidence fully. Hair strand analysis should instead form part of carefully considered expert opinion evidence.

FTS does not rely solely on Society of Hair Testing (SoHT) cut-off levels when interpreting results. Numerical thresholds can oversimplify findings if considered without the wider context. Our approach is to report all relevant findings and explain what they may mean within the circumstances of the case, including what the evidence can and cannot demonstrate. This allows the findings to be considered appropriately within the context of the balance of probabilities and provides those making decisions within proceedings with balanced expert opinion evidence.

Asking the right question

The question “how far back does a hair strand drug test go?” has no single answer. While sufficient scalp hair may provide a window of up to 12 months, that timeframe is approximate and does not mean drug use can be assigned to exact dates.

More importantly, the length of the testing window is only one consideration. What matters is whether the available sample, analytical approach and expert interpretation can provide meaningful evidence in relation to the circumstances of the case.

Hair strand testing can provide valuable evidence of historical drug use and patterns over time, but it should not be expected to provide certainty beyond what the science supports. When hair testing is considered alongside the wider evidential picture and other relevant evidence, it can help inform a more balanced understanding of an individual’s drug use over the period examined.

Do you require 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.

You can request a quote, make an instruction or speak to a member of the FTS team via email: This email address is being protected from spambots. You need JavaScript enabled to view it. or call 01924 480272. 

Forensic Testing Service (FTS) is the UK’s specialist provider of court-approved drug, alcohol and DNA testing for family law, social care and safeguarding cases.