How Far Back Can a Hair Drug Test Go?
Ninety days is the standard, but the timeline can be extended. How sample length maps to detection window.
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Hair grows at a steady enough rate to work as a record. Each centimetre closest to the scalp corresponds to roughly a month, which is what allows a hair test to look backwards rather than only at the present. The question is how far back that record reliably goes.
How the record forms
When a drug is taken, the body breaks it down into metabolites that circulate in the bloodstream. As hair is formed in the follicle, both the drug and its metabolites are incorporated into the growing strand and stay there.
That is the reason a hair result reports both the parent drug and its metabolites: the metabolite is the part that can only have come from inside the body.
The standard window: 90 days
Most tests analyse the 3 cm of hair closest to the scalp. At an average growth rate of 1 cm per month, that section covers approximately 90 days.
| Sample length | Approximate window |
|---|---|
| 3 cm | 3 months |
| 6 cm | 6 months |
| 9 cm | 9 months |
| 12 cm | 12 months |
Longer samples are analysed in 3 cm segments rather than as one length, because a single long sample averages everything into one figure and loses the timeline. There is more on that in can hair testing go back 12 months.
Growth rate varies between individuals — roughly 0.7 to 1.5 cm per month — so every one of these windows is an approximation, not a set of dates.
Body hair
Where scalp hair is not available, body hair can be used. It grows more slowly and sheds less often, so it can cover a longer period — sometimes 12 months or more.
The trade-off is precision. Body hair growth cycles are irregular, with a higher proportion of follicles dormant at any time, so the window it represents is much less exact. As a rule of thumb, add about three months: 3 cm of body hair may represent six months rather than three.
What a hair test cannot tell you
It cannot date use to a particular day. It cannot state how many times a substance was taken.
What it can do is report the concentration, and concentration carries information — higher levels are generally associated with heavier or more frequent use, though a short period of heavy use inside a longer window complicates that reading. Interpreting the number is a separate exercise from measuring it, and what the levels mean sets out how it is done.
Laboratory wash protocols and metabolite reporting are what keep external contamination — second-hand smoke, handling — out of the result.
Why the method holds up
Hair analysis is difficult to defeat and covers a period no other routine sample can. That combination is why employers, courts and treatment programs rely on it where the question is a pattern of behaviour rather than a single moment. The comparison with urine testing is the clearest way to see the difference.
Still have questions?
Detection windows, what a positive result actually means, and how a sample is collected are all covered on our FAQ page. If you know what you need, book a test.
