Hair testing is popular in custody cases because it looks back further than urine. Here’s what that window really covers.
The standard window: about 90 days
Head hair grows about half an inch per month on average. Labs usually test the 1.5 inches closest to the scalp, which represents roughly the last 90 days.
Growth rates differ between people, so the window is an estimate. Some people’s hair grows faster and some slower, which means the same length of hair can represent a slightly shorter or longer period.
How drugs get into hair
When a person uses a drug, it circulates in the blood and is deposited into the hair as it forms in the follicle. As the hair grows out, it carries a record of what was in the bloodstream at the time.
Why it doesn’t show the last few days
After drug use, it takes about a week for the affected part of the hair to grow above the scalp where it can be cut. So hair tests generally don’t detect very recent use.
Body hair
If head hair is too short, body hair can be used. Body hair grows differently, so the window is less precise and may cover a longer period.
Segmented testing
Some labs can cut the hair into segments and test each one separately. This can help show whether use increased, decreased or stopped over time.
Segmented testing is useful when a parent says they stopped using on a particular date. It can support or contradict that claim, though the timing is still approximate.
Hair treatments
Bleaching, dyeing and chemical straightening can reduce drug levels in hair. Labs take this into account, and some note visible treatment on the report. A person shouldn’t treat or cut their hair before a test, and doing so right before a court-ordered test may itself raise questions.
What hair tests are good for
- Showing a pattern of use
- Checking claims of long-term abstinence
- Complementing short-window urine tests
What they’re not good for
- Proving use on a specific day
- Detecting very recent, one-time use
How much hair is needed
The collector cuts a small bundle of hair, about the thickness of a pencil, close to the scalp, usually from the back of the head where it’s less noticeable.
Can hair testing show a single use?
Hair testing is designed to detect repeated or regular use. A single use may or may not be detected, depending on the drug, the amount and the lab’s cutoff. That’s why a negative hair test is strong evidence against ongoing use but can’t completely rule out an isolated occasion.
External contamination
Drug smoke or residue can land on hair from the outside. Labs wash samples before testing and, for many drugs, look for metabolites that the body produces only after use. These steps help distinguish a person who used drugs from one who was simply around them.
Common questions
- Does shampoo or washing remove drugs? Normal washing doesn’t remove drugs deposited inside the hair shaft.
- Will the cut be noticeable? The collector takes a small bundle from the back of the head, close to the scalp. It’s rarely visible.
- Can braids, weaves or extensions be tested? Only the person’s own hair can be used. The collector may take hair from between braids or from another area.
The bottom line
A standard hair test is the best single test for showing whether someone has used drugs regularly over roughly the past three months. Pair it with a urine test when the most recent days also matter.
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This article is general information, not legal or medical advice. Testing requirements vary by court and agency.