Hair drug and alcohol testing: the complete guide
Hair testing reveals a pattern of drug or alcohol use over months, not days. This guide explains how it works, how far back it can look, how a timeline is built, and where the method has limits.
How it works
Why hair holds a history
As hair grows, substances circulating in the bloodstream are incorporated into the hair shaft and locked in place. Because head hair grows at a fairly steady rate, that shaft becomes a timeline: the hair nearest the scalp is the most recent, and each further centimetre reaches further into the past. This is what allows hair testing to show a pattern of use over an extended period.
How far back it looks
Head hair grows at approximately one centimetre per month. A standard 3 cm sample therefore reflects roughly the previous three months. Longer samples can be analysed to look back further, subject to hair length.
Building a timeline with segmentation
Where a case needs to know not just whether, but when, the hair can be cut into segments and each segment analysed separately. A 3 cm sample split into three 1 cm sections gives an approximate picture of when use took place across the period, which can show whether use was recent, historic, increasing or decreasing. Timing is an estimate, not an exact date. This is particularly valuable in family and care proceedings, where the timing of use matters as much as its presence.
The lag before recent use appears
Hair testing has one important limitation to understand: it does not reflect the last few days. Newly grown hair takes time to emerge above the scalp where it can be sampled, so very recent use will not yet be visible in a hair test. Where recent use is the question, an oral fluid or urine test is the right tool.
This page is a general guide. It is not legal, medical or technical advice. Laws, guidance and laboratory methods change, so check the current requirements for your own case. Detection windows are indicative. They vary with the substance, dose, how often it is used and the individual. Last reviewed .
Hair alcohol testing: EtG and FAEE
Hair can also show a pattern of alcohol use over months. The main marker is EtG, and a second marker can be added:
- EtG (ethyl glucuronide) — a direct metabolite of alcohol, incorporated into the hair from sweat and the bloodstream.
- FAEEs (fatty acid ethyl esters) — a separate group of alcohol markers formed by a different process, which can be added to strengthen the picture.
A low EtG result is consistent with abstinence or light drinking but does not rule out occasional drinking. A raised result counts against abstinence. Bleaching, dyeing and perming can reduce both markers, and hair products containing ethanol can raise FAEE, so please tell the collector about any treatments. Results are read alongside the case history, and are used to support or test accounts of drinking over a period of months.
The limits of hair testing
Hair testing is powerful, but it is not the answer to every question:
- It is not for recent use. Because of the growth lag, it cannot confirm use in the last few days.
- Cosmetic treatment can affect it. Heavy bleaching in particular can lower the level of some substances and can make a negative result less reliable, so treatments must be disclosed and are taken into account.
- Sample availability. Where scalp hair is too short or unavailable, body hair can often be used, but it reflects a longer, less precise window. Body hair can sometimes be used for alcohol too; your collector will advise.
- Single low-level events. Hair testing reflects patterns of use; it is less suited to detecting a single isolated exposure.
When to choose hair testing
Hair testing is the right choice when the question is a pattern over time rather than recent use — most commonly in court and safeguarding cases, in child-protection assessments, and where a statement of abstinence needs to be checked. It is frequently instructed alongside a urine test so that a single instruction evidences both a recent-use snapshot and a longer-term pattern. To compare it against other sample types, see choosing the right sample.
Common questions
Hair testing — your questions answered
How far back can a hair drug test detect use?
As a rule of thumb, head hair grows about one centimetre a month, so a 3 cm sample reflects roughly the previous three months. Longer samples can be analysed to look back further, and can be segmented to build a month-by-month timeline of use.
How much hair is needed for a test?
Only a small sample is needed — a lock of hair about the width of a thin pencil, taken from the back of the head and cut close to the scalp. It is taken discreetly and is not noticeable afterwards. Where head hair is unavailable, body hair can often be used instead, though it reflects a longer and less precise time window.
Can hair testing show alcohol consumption?
Yes. Hair alcohol testing measures EtG, and can add a second marker, FAEE, to strengthen the picture. Results show the pattern of drinking over a period of months and are read alongside the case history. A low EtG result is consistent with abstinence or light drinking but does not rule out occasional drinking.
Does hair colour, dye or bleaching affect the result?
Bleaching, dyeing and perming can reduce both alcohol markers, and hair products containing ethanol can raise FAEE. Please tell the collector about any treatments — the laboratory takes them into account when interpreting the result. Hair colour itself does not prevent testing.
Can hair testing support abstinence?
Hair testing is well suited to supporting a statement of abstinence over a defined period, which is why it is widely used in family and care proceedings. A negative result across a segmented sample is consistent with non-use across those months. A low EtG result is consistent with abstinence or light drinking but does not rule out occasional drinking. A raised result counts against abstinence. Results are read alongside the case history.
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