How hair testing works
When THC is in your bloodstream, trace amounts of its metabolites are incorporated into the hair as it grows from the follicle. A lab takes a small sample cut close to the scalp, dissolves it, and screens for those metabolites. Because hair grows at a fairly steady rate of roughly 1 cm per month, the length of the sample maps to a span of time.
Where the 90-day number comes from
The standard test uses about 1.5 inches (3.8 cm) of hair measured from the scalp, which corresponds to roughly the most recent 90 days of growth. That's the source of the "three months" figure. It's a history test by design — built to reveal a pattern of repeated use rather than a single instance.
The calculator includes hair testing so you can understand the window you're actually dealing with.
Run my estimate →The recent-use blind spot
Here's the nuance: it takes time — often 5 to 10 days — for newly grown hair containing metabolites to emerge above the scalp where it can be sampled. That means a hair test is paradoxically weak at catching use from the last several days, even as it reliably catches use from weeks or months ago. It's the mirror image of a saliva test, which only sees the very recent past.
Do detox shampoos work?
This is the hair-test equivalent of the detox drink, and the evidence is similar: unconvincing. Because metabolites are built into the hair shaft from the inside as it grows, a topical product washing the outside has limited ability to remove them, and labs wash samples before analysis to strip surface contamination anyway. Aggressive stripping routines also tend to visibly damage hair, which can itself raise questions. There's no reliable, evidence-backed way to "wash out" an internal record.
What to expect
- ›Hair tests reward time more than tricks. The only thing that shrinks your exposure window is a longer gap since last use.
- ›Body hair may be used if head hair is unavailable, and it can reflect a longer span.
- ›It's a pattern test. A single distant exposure is less likely to register than consistent use.
- ›Confirmation matters. Non-negative screens are confirmed with more specific lab methods.
If you've been told a hair test is coming, it's worth understanding that it's the hardest of the common tests to influence in the short term — which makes planning around your real timeline, and knowing your rights, all the more important.
An independent desk reviewing pharmacokinetic research and regulatory guidance from SAMHSA and DOT. Content reviewed quarterly for accuracy.