How Long Does It Take to Pass a Hair Follicle Test? The 90-Day Rule, Hair Growth Math and Body Hair

Updated 16 Oct 2026 · By the DrugTestAnswers editorial team

Straight up: for most people, a standard hair test looks back about 90 days, so the honest answer is roughly three months after your last use, sometimes a bit longer. There's no quick fix here, and anyone who tells you otherwise is selling something. Here's the math behind that number, why body hair is different, and how you can check where you stand.

Why hair tests look back 90 days

When you use a drug, small amounts travel in your blood to the hair follicle and get locked into the hair shaft as it grows. That hair then pushes out of the scalp at about half an inch (roughly 1.25 cm) per month. Labs cut a 1.5-inch sample from as close to the scalp as possible. Do the math: 1.5 inches divided by 0.5 inches per month is about three months. That's where the famous 90-day window comes from, and it's the standard described in SAMHSA's guidance and by the Society of Hair Testing.

It also explains the flip side: hair takes about 7 to 10 days to grow out far enough above the scalp to be cut. So drug use in the last week or so usually won't show up in hair yet. That's why hair tests are poor at catching very recent use, and why some employers pair them with a urine test.

So when is "clean"?

If your last use was more than about 90 days ago, the 1.5-inch sample should mostly be hair that grew after you stopped. Some practical wrinkles:

• Growth rate varies. Some people grow hair at 0.4 inches a month, some at 0.6. Slower growers can carry an older timeline in that 1.5 inches.
• The lab may cut a longer sample or test segments, though standard pre-employment tests use 1.5 inches.
• Heavy, frequent use deposits more drug, so the tail end of your using period may still be above the cutoff if your hair grows slowly.
• Single use is often below hair cutoffs to begin with. Hair tests are best at catching repeated use; detecting one-off use, especially of THC, is unreliable.

A cautious rule of thumb many people use: give it 90 days, and more like 100-120 days if you were a daily user or your hair grows slowly.

Body hair is a different clock

If your head is shaved or your hair's too short, labs collect body hair: chest, armpit, arms or legs. Body hair grows more slowly and has longer resting phases, so it can reflect a much longer window, often quoted as up to about a year. Shaving your head doesn't make a hair test go away; it usually just means body hair gets tested instead, which can be worse for you.

Which drugs show best in hair?

Hair tests are very good at catching cocaine, amphetamines, methamphetamine, MDMA and opioids because those compounds bind well into the hair shaft. THC is the weakest. The main metabolite, THC-COOH, is incorporated in tiny amounts, so hair cutoffs for THC are extremely low (picograms per milligram). Daily cannabis users are usually caught; occasional users often aren't. Darker hair, which contains more melanin, tends to bind some drugs (especially basic drugs like cocaine and amphetamines) more strongly, a known fairness concern in hair testing.

What about detox shampoos?

You'll see plenty of shampoos marketed for hair tests, along with home routines involving vinegar, bleach and detergent. Labs wash hair samples before testing to remove outside contamination, and there's no independent, peer-reviewed evidence that consumer shampoos reliably remove drug that's locked inside the hair shaft. Harsh chemical routines can also damage hair and scalp. The best-known product here is Old Style Aloe Toxin Rid; see our Aloe Toxin Rid review and how-to-use guide for what it claims and what the evidence says, but the real answer to "how long" is still time.

Check yourself with a home hair test

A mail-in home hair test lets you send your own sample to a lab and see the result privately. It's the only way to know whether your timeline has really cleared before an employer test. If it comes back positive, you've learned something important without consequences. If it's negative, you can stop worrying.

Quick timeline table

These are estimates, not guarantees. See our sourced detection table and the detection window calculator.

Know the limits

If your test is part of a DOT, federal, safety-sensitive or court process, don't try to work around it. Talk to the program, a lawyer or a counselor. And if quitting for 90 days feels impossible, that's worth paying attention to. The SAMHSA National Helpline (1-800-662-4357) is free and confidential.

A month-by-month picture

Here's a simple way to visualize your hair sample. Imagine the 1.5 inches split into three half-inch segments. The half-inch nearest your scalp is roughly the last month, the middle segment is the month before, and the far end is the month before that. If you stopped using on day zero:

• After 1 month: the newest segment is clean, but the other two still reflect your using period. A test would very likely be positive for regular users.
• After 2 months: two segments are clean. The oldest segment still carries drug. The overall result depends on how much is in that last segment versus the cutoff, since the lab grinds the whole sample together. Daily users are usually still positive.
• After 3 months: the whole 1.5-inch sample should have grown after you stopped. This is the point most people aim for.

Because the lab mixes the whole sample, two clean months can sometimes dilute one dirty month below the cutoff for light users. That isn't reliable enough to bet a job or a paycheck on, though.

Does hair color or treatment change the window?

Bleaching, dyeing and perming can reduce drug levels in hair somewhat, and research has documented this. But labs know about it, may notice chemically treated hair, and the reduction is inconsistent. It doesn't change the 90-day timeline, and repeated harsh bleaching can wreck your hair. We wouldn't recommend relying on it.