What affects a hair drug test result?

Court-ordered drug tests are performed in Ireland every single day for a variety of reasons, including making decisions about custody disputes or a child’s welfare.

One of the most sophisticated types of drug test is hair drug testing, which is also known as hair strand testing or hair follicle testing.

Here, we explore hair drug testing in more detail, including how it works and whether there are external factors that could affect the hair drug test result.

What is hair drug testing?

A hair drug test, also known as a hair strand test or a hair follicle test, involves analysing 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, and the parent drug and a proportion of the drug’s metabolites enter the hair follicle. When the hair is subjected to analysis, the parent drug and its metabolites can be identified within the hair.

Hair drug testing is routinely used within family law and forensic fields and is renowned as being the gold standard in toxicological analysis. At AlphaBiolabs, our hair drug testing is 100% accurate and reliable, based on the hair samples received at our laboratory.

How does head hair drug testing work?

Head hair drug testing requires a cut hair sample to be analysed for the presence of drugs and their metabolites. When drugs are consumed and enter the system, part of  the drug enters the hair follicle and is trapped in the innermost layer of the hair shaft, the medulla.

It takes around 7-10 days for the hair follicle containing the drug to grow above surface level and a further two weeks for the 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 certain periods, but this depends on the length of hair in the sample.

There are two types of head hair drug testing: segmented analysis and overview analysis.

Segmented analysis, which is the most detailed type of hair strand testing, involves segmenting the hair sample into 1cm sections for a month-by-month profile. This method can provide a detection window of up to 12 months and is favoured by those who need to identify a trend in drug use.

Overview analysis offers a more general insight into an individual’s drug use. In this method, the maximum length of hair that can be analysed is 3cm, which provides an overview of 3 months. This can be repeated up to four times to give a detection window of up to 12 months. However, rather than one 12-month overview, the 4 x 3-month overviews are reported separately.

The difference between segmented analysis and overview analysis is that segmented analysis enables you to determine the exact month a drug was consumed. Overview analysis only determines whether or not drug markers are present in a particular sample.

How does body hair drug testing work?

If head hair is not available for drug testing, body hair can be collected – from the face, chest, leg, arm, back or underarm.

Body hair grows at a different rate compared to head hair, and its growth isn’t consistent, which means it cannot be segmented to offer a month-by-month profile. Instead, body hair can only be used to provide up to a 12-month overview.

When body hair samples are collected, they are done so by weight rather than hair length.

How are hair samples collected and tested for hair drug testing?

For hair drug tests to be admissible in court, hair samples must be collected by a trained sample collector under strict chain of custody conditions to prevent tampering.

A head hair sample  will be cut from an area around the highest point on the scalp, as this area has the least variation in growth rates. Samples must contain around 200 strands of hair and two samples are collected.

The samples are then transported to the laboratory and testing can begin. At AlphaBiolabs, all hair samples are washed three times to remove any surface drugs. This is because if someone has been in an environment that is heavily laden with drugs or they have been around someone else who has been smoking drugs, traces of the drug could be on their hair. Drug  traces can also be transferred onto hair by direct contact. The washing solution is kept and can be analysed if required.

What affects a hair drug test result?

A hair drug test can confirm whether or not an individual has consumed drugs during a certain period. However, it cannot tell us the exact date, time, or method of ingestion.

There are certain factors that have the potential to impact the result of a hair drug test. For this reason, reputable laboratories like AlphaBiolabs will ask for full disclosure of any recent hair and beauty treatments, as well as current medical conditions and medications, so that this can be considered.

Some of the most common factors known to affect hair drug test results include:

Hair treatments

Bleaching, dyeing, perming, and straightening can all impact a hair drug test result. This is because the chemicals used in these types of treatments can damage the hair shaft and alter the concentration of the drug.

Hair colour

Studies show that the more melanin in the hair, the more likely that weak base drugs will bind to the hair. So, drugs like opioids, cocaine and methamphetamine bind more easily to darker hair, which results in higher concentrations of the drug in tests.

Hair growth rate and length of hair available

Hair drug tests can either analyse head hair, body hair or facial hair. Depending on the length of the hair being analysed, drugs can be detected up to 12 months after when the drugs were consumed.

The Society of Hair Testing (SoHT) advises that scalp hair from the posterior vertex should be the preferred head hair sample, as it offers the highest percentage of follicles in the anagen phase and the most consistent growth rate.

Scalp hair grows at an average rate of 1cm every month, meaning a 3cm sample can provide up to a 3-month window for drug testing.

Body hair and facial hair have a much more variable growth rate and more variable resting cycles compared to scalp hair, meaning that they can only provide up to a 12-month overview of drug use.

Environmental factors

Passive exposure to a drug through hands or smoke could potentially present a false positive reading. At AlphaBiolabs therefore, our rigorous hair drug testing processes include the decontamination of hair samples through a three-step chemical wash. The washed liquid is also kept at our laboratory for further analysis if required.


For hair drug tests to be admissible in court, hair samples must be collected under strict chain of custody conditions to reduce the likelihood of tampering. However, some individuals will still attempt to cheat a hair drug test, which is why professionally-trained sample collectors must always be used to collect a sample.

Common ‘cheating’ methods include wearing a wig, trying to use someone else’s pre-cut hair, and even shaving off all body and head hair. AlphaBiolabs is a UKAS-accredited laboratory and only employs professional sample collectors, so you can rest assured that any attempts to cheat a hair drug test will be completely ineffective.

Where can I get a hair drug test?

AlphaBiolabs has extensive experience providing hair drug testing services for local authorities, family law professionals and social workers. We have invested in the very latest technology for our state-of-the-art toxicology laboratory, employing the very best scientists, so you can be confident of receiving a 100% accurate result.

We have a nationwide network of professional sample collectors, and all sample collections are performed under strict chain of custody conditions.

We also offer free sample collection for legally-instructed hair drug tests from our Dublin walk-in centre.

For expert advice on hair drug testing, or to request a quote, call AlphaBiolabs’ Legal team on 0140 29466 or email

Alternatively, complete our online quote form, and a member of the team will be in touch to discuss your needs.

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