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How far back can a hair drug test show?
By Karolina Baker, Health Testing Specialist at at AlphaBiolabs
Last reviewed: 27/01/2025
A hair drug test can be used to detect historic drug use for a period spanning up to 12 months, depending on the length of hair available.
Segmented analysis can identify patterns in drug use for a period of up to 12 months. For this type of analysis, the hair is segmented into 1cm sections, for a month-by-month profile.
Overview analysis provides an overview of drug use for the period being tested. The maximum length of hair that can be tested is 3cms (overview of 3 months), with the maximum overview period being 12 months (reported as 4x 3-month overviews).
In this article, we look at court-admissible hair drug testing in more detail.
What is hair drug testing?
A hair drug test, also known as a hair strand test or a hair follicle test, is used to analyse samples of head hair or body hair for the presence of drugs and their metabolites.
When a person consumes drugs, they are passed into the bloodstream. Through the bloodstream, a proportion of the parent drug and its metabolites enter the hair follicle and become entrapped within the hair shaft as it grows.
Therefore, when the hair is analysed at the toxicology laboratory the parent drug and/or its metabolites can be identified within the hair.
How does hair drug testing work?
Hair drug testing works by analysing a cut hair sample for the presence of drugs and their metabolites.
When a person consumes drugs, they are broken down by the liver, and a proportion of the parent drug and its metabolites are absorbed into the bloodstream.
These drugs then enter the hair follicle and become trapped in the medulla, the innermost layer of the hair shaft, making it possible to detect them during analysis at the toxicology laboratory.
It takes approximately 7-10 days for the hair containing the drugs to grow above the surface of the scalp, and two weeks for this hair to be included in a cut sample.
The rate at which head hair grows enables us to determine a pattern of drug use over a defined period, depending on the length of hair selected (1cm of hair = one month of growth).
There are two methods available for head hair drug testing: segmented analysis or overview analysis.
Segmented analysis is the most detailed form of hair strand testing for drugs. This involves segmenting the hair into 1cm sections, for a month-by-month profile and therefore, a more detailed history of drug use.
This method is particularly useful if you need to identify a detailed trend in drug use, such as an increase or decrease over time, or to highlight intermittent use.
Segmented analysis can be used to identify patterns in drug use for a period of up to 12 months. The table below demonstrates how segmented analysis can be used, and how many segments of hair would be required depending on the period being tested.
| Segmented analysis required (months) | Number of segments (1cm per segment) | Reported time period |
| 1 | 1 | 1 month |
| 3 | 3 | 3 individual months |
| 6 | 6 | 6 individual months |
| 9 | 9 | 9 individual months |
| 12 | 12 | 12 individual months |
Overview analysis is beneficial if you are looking to obtain a more general insight into a person’s drug use.
This is because any episodes of drug use are averaged out over the period being tested.
The maximum length of hair that can be tested to provide an overview of drug use is 3cms, which provides an overview of three months.
The maximum overview period that can be provided is 12 months. However, this would be reported as 4 x 3-month overviews, rather than one 12-month overview.
The table below shows the options available for hair drug testing using overview analysis, and what will be shown in the report, depending on the required overview period:
| Overview analysis required (months) | Number of segments (3cm per segment) | Reported time period |
| 3 | 1 | 3 months |
| 6 | 2 | 2 x 3-month periods |
| 9 | 3 | 3 x 3-month periods |
| 12 | 4 | 4 x 3-month periods |
It is important to consider that in a hair strand overview test, an individual could test positive if they had abstained from drugs for two out of three months, but ingested an excessive quantity in the other month, as the overview test averages out the consumption over the 3-month period.
Unlike with segmented analysis, it is not possible to specify in which month the drugs are likely to have been consumed. The test will only show whether or not drug markers are present within the sample covering the 3-month period.
How far back can a hair drug test show?
Segmented analysis can be used to identify patterns in drug use for a period of up to 12 months. However, this may be limited by factors such as the length of hair available and differences in hair growth rates.
Overview analysis is beneficial if you are looking to obtain a more general insight into a person’s drug use. This is because any episodes of drug use are averaged out over the period being tested.
The maximum length of hair that can be tested to provide an overview of drug use is 3cms, which provides an overview of 3 months.
The maximum overview period that can be covered is 12 months. However, this would be reported as 4 x 3-month overviews, as opposed to 1 x 12-month overview.
What factors are considered when compiling a hair drug test report?
The results of a hair drug test should never be interpreted in isolation. Several factors can influence how drugs are incorporated into the hair, meaning expert analysis is essential to ensure a fair and accurate assessment.
Factors that AlphaBiolabs’ Reporting Scientists will always consider when reporting hair drug test results, include:
- Hair type and colour – melanin levels in hair can impact drug incorporation, meaning darker hair may sometimes retain higher concentrations of certain substances.
- Cosmetic treatments – chemical treatments such as excessive bleaching, dyeing, perming or straightening have the potential to affect drug retention in hair strands.
- External contamination – environmental exposure to drugs (e.g. through smoke or physical contact) does not necessarily indicate use. Our reports consider the presence of metabolites – byproducts produced by the body after drug use – to help distinguish between active consumption and contamination. We also ensure the hair being tested is thoroughly washed first to ensure any surface contaminants are removed/minimised.
- Biochemical properties of drugs – different drugs bind to hair at different rates, meaning some substances are more likely to be retained than others.
- Variable growth rates of hair – Hair typically grows at an average rate of approximately 1 cm per month, as referenced by the Society of Hair Testing (SoHT). However, this rate can vary between individuals due to factors such as age, genetics, medical conditions, and ethnicity, potentially affecting the approximate time period represented by each hair segment.
What’s included in an AlphaBiolabs hair drug test report?
We are committed to delivering scientifically robust and transparent drug test results.
Our reports provide context and expert interpretation, ensuring that results are not taken at face value but considered holistically.
Our hair drug test reports include:
- Sample donor information – including photo ID, and details of any disclosures made at the time of the sample collection (e.g. prescribed medications, drug/alcohol usage, environmental exposure to illicit drugs/close body contact with a drug user).
- Substances found and detected levels – which drugs have been detected, along with their concentration levels, measured in nanograms per milligram (ng/mg) or picograms per milligram (pg/mg) of hair (analyte dependent).
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Comparison with internationally-recognised cut-off levels – to ensure accuracy, the levels of substances detected are compared against scientifically validated thresholds. These cut-off levels play a fundamental role in helping to distinguish between actual drug use and potential external contamination.
AlphaBiolabs also has analytical cut-offs for each analyte that are higher than the SoHT cut-offs to take into account analytical variation. Therefore, when a positive result is reported, there is no doubt that the compound is present within that tested sample.
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Interpretation of the test results, including relevant caveats and considerations – a drug test report should not simply tell you whether the sample donor has tested positive or negative for a certain substance.
Our reports include expert analysis, considering factors such as possible external exposure to illicit substances, sample donor declarations, the presence of metabolites (which indicate active drug use), pattern assessments of active and previous drug use, and consideration of the natural variations in hair drug incorporation and rates of hair growth.
- Segmented analysis – if requested, hair samples can be segmented to provide an insight into a pattern of drug use over time, rather than a single result for the entire period (overview analysis).
- Contextual considerations – factors such as hair treatments, hair colour, sample donor declarations (such as prescribed medications/declared substance use), and the donor’s ethnicity are all carefully considered, to ensure a fair and accurate interpretation.
Segmented (month-by-month) hair drug testing
Segmented analysis can be used to establish patterns of drug use over a defined period. This is because head hair grows at an average rate of 1 cm per month.
Therefore, a month-by-month summary of drug use can be determined by segmented hair strand analysis.
Segmented hair strand analysis can be used to identify trends of drug use for a period of up to 12 months, depending on the number of 1 cm sections of hair available and/or selected.
Overview hair drug testing
Overview analysis is helpful in obtaining a general indication of drug use. It can be performed using as little as 3cm of hair to identify drug use within that 3-month period, and up to 12 months, depending on the length of hair available.
Another option is a 6-month overview split into two results: one for the first 3 months and a second result for the second 3 months.
If a more detailed analysis is needed, segmented would be the preferred option.
Where can I get drug hair test?
- AlphaBiolabs has extensive experience providing hair drug testing services for local authorities, family law professionals and social workers
- All hair sample collections are performed under strict chain of custody conditions, and our test results are accepted by courts in Ireland
- We also offer free sample collections for legally-instructed hair drug tests from our Dublin walk-in centre
For expert advice or to request a quote for hair drug testing, call our Legal team on 01 402 9466 or email testing@alphabiolabs.com
Alternatively, complete our online quote form, and a member of the team will be in touch to discuss your needs.
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Karolina Baker
Health Testing Specialist at AlphaBiolabs
Karolina joined AlphaBiolabs in 2021, and holds the role of Health Testing Specialist.
As well as overseeing a range of health tests, Karolina plays an active role in the research and development of the company’s latest health test offerings.
Before joining AlphaBiolabs, Karolina worked as an Associate Practitioner at Mid-Cheshire Hospitals NHS Foundation Trust, and as a research assistant at the Turner Laboratory, within the Faculty of Biology, Medicine and Health at The University of Manchester.
Karolina’s main scientific interests include clinical genomics and genetic diagnostics. Her qualifications include a BSc in Molecular Biology and an MSc in Genomic Medicine.
