How Far Back Does a Hair Drug Test Go? 90 Days, 1 Year, 7 Years and What Labs Actually Cut

Updated 20 Oct 2026 · By the DrugTestAnswers Editorial Team

A standard hair drug test goes back about 90 days. That number isn't a legal limit or a chemical one. It comes from simple growth math: the lab cuts roughly the first 1.5 inches (about 3.9 cm) of hair closest to your scalp, and head hair grows close to half an inch a month, so that strip holds around three months of history. Longer hair, body hair and special lab requests can stretch the window, while the newest week or so usually isn't visible yet. Here's how each piece works, so you can figure out what your own sample is likely to show.

The quick answer by sample type

Most people being tested for a job fall into the first row. If the collector cuts head hair and doesn't mention anything unusual, plan around the 90-day figure.

Why the number is 90 days

Drugs and their breakdown products travel in your blood. Blood vessels feed the hair follicle under the skin, so a little of whatever is circulating gets locked into the hair as it forms. That hair then pushes up and out of the scalp at a fairly steady rate. On average, scalp hair grows about 1 to 1.3 cm a month, which is close to half an inch.

Testing labs settled on a sample of about 1.5 inches cut as close to the skin as possible. Three months of growth fits in that strip, so the result reflects about the last three months. The figure is an average. Fast growers pack slightly less than 90 days into the same length, and slow growers pack a little more. Hair also grows in cycles, and at any moment a share of your scalp hairs are resting rather than growing, which blurs the timeline at the edges by a few weeks.

The most recent week usually isn't there yet

New hair takes time to grow from the follicle up past the skin, where the scissors can reach it. That's why a hair test is a poor tool for very recent use. Drug use from the last 5 to 10 days typically hasn't surfaced in a cuttable section. This gap is also why some employers pair a hair test with a urine test, since urine covers the last few days that hair misses.

It doesn't make the last week a safe zone in any practical sense. Most people asking this question used over a longer stretch, and anything from about a week ago through roughly three months ago sits squarely inside the sample.

Can a hair test go back a year?

It can, but not with a standard sample. If your hair is long and the requester pays for it, a lab can test a longer section. Each extra half inch adds about a month. Hair that's 6 inches long holds roughly a year of growth.

There are limits. The older end of a strand has been washed, brushed, heated and exposed to sunlight for months, and those things slowly lower drug levels. Results from the far end are weaker and harder to interpret, so most labs and employers stick with 1.5 inches. Extended windows show up more in family law, treatment programs and forensic cases than in routine hiring.

What about "7 years" or "forever"?

You'll see claims online that a hair test can go back seven years or more. In theory, drug traces stay in a strand of hair for as long as that strand exists, so very long hair technically carries a long record. In practice, no ordinary workplace test looks at hair that old. The sample is cut at the scalp and the lab only analyzes the length it's asked to test. Once you've cut your hair or it has grown out past the tested section, older use isn't part of the result.

Researchers have analyzed very long hair in special studies, including historical and archaeological samples. That's a different world from a $100 to $200 pre-employment panel.

Body hair stretches the window

If you have little or no hair on your head, the collector will usually take hair from your chest, arms, legs or underarms. Body hair grows more slowly than scalp hair and spends much longer in a resting phase. A single clipping can contain hairs that formed at very different times, so the sample may reflect up to about a year of history rather than three months.

That also means shaving your head right before a test rarely helps. Our guide to which body hair can be used for drug testing covers each body site and its rough window, and the page on a hair drug test with a shaved head explains what collectors do when there's nothing on top.

Segment testing: when the lab splits the timeline

A normal test treats the whole 1.5 inches as one sample and gives one result. In segmental analysis, the lab cuts a longer strand into pieces, often half-inch or one-inch sections, and tests each one separately. Because each section matches roughly one or two months of growth, the report can show whether use was recent, older or ongoing.

Segment testing is more expensive and slower, and even then it gives a rough calendar, not exact dates. You'll mostly find it in custody disputes, monitoring programs and cases where someone claims they stopped using at a certain point.

How far back for each drug

The look-back is about the sample, not the drug, so the same 90-day strip applies to every substance on the panel. What changes from drug to drug is how much of it ends up in the hair.

• Cocaine, meth and amphetamines bind strongly to hair. Repeated use within the window is very likely to show, and limited use often shows too.
• Marijuana leaves much less in hair. The metabolite labs look for gets in at tiny amounts, so a single use often falls under the cutoff, while daily use usually doesn't.
• Opioids fall in between. Heroin, codeine and morphine show up reasonably well; some synthetic opioids need an expanded panel.
• Prescription drugs like Adderall or benzodiazepines can appear if the panel includes them. A valid prescription is handled by a medical review officer after the lab result.

For drug-specific detail, see whether a hair test can detect weed, detect meth or detect opiate use.

Things that shift your personal window

• Growth rate. Age, genetics, pregnancy, thyroid problems and nutrition all change how quickly hair grows, so your 1.5 inches may cover a bit more or less than three months.
• Hair color. Many drugs bind to melanin, the pigment in hair, so darker hair tends to hold more drug for the same use. It doesn't change the timeline, only how strong the signal is.
• Chemical treatments. Bleach, perms and relaxers damage the hair and can reduce drug levels somewhat. They don't erase the timeline and they leave visible damage the lab may note. Our pages on hair dye and bleach and perms look at what studies found.
• How much and how often you used. Heavy, repeated use builds higher levels across the whole strip. Light use may leave some months with traces under the cutoff.

Working out your own timeline

Count back from your test date. If your last use was more than about 90 days before a standard head-hair test, the hair grown since then is what the lab will most likely cut. If your last use was 30 days ago, the sample contains roughly two months of hair grown while you were still using, plus one month after. Body hair adds uncertainty, so allow several more months if that's what will be collected.

Our article on how long it takes to pass a hair follicle test walks through month-by-month examples if you want to see the math laid out.

Check before the real test

Guessing from averages only gets you so far. A home hair test kit lets you snip a sample yourself, mail it to a lab and get a private result back, using the same kind of screening an employer uses. It's the clearest way to see where you stand before a test you can't redo. If you're comparing options, our roundup of the best hair follicle drug test kits explains what to look for.

Product facts: Aloe Toxin Rid and the 90-day window

Because a hair test reaches so far back, many people start reading about detox shampoos once they realize waiting may not fit their schedule. The most searched one is Old Style Aloe Toxin Rid, sold by TestClear. TestClear says it's designed to cleanse the hair shaft and recommends a routine of repeated washes over several days, often paired with Zydot Ultra Clean on the day of collection. Those are the maker's directions and claims. No independent, peer-reviewed research shows that any shampoo reliably clears drug residue from inside the hair, and none can promise a pass. If you'd like to read the maker's own instructions and pricing, the Old Style Aloe Toxin Rid product page has them, and our Aloe Toxin Rid review and how-to-use guide separates the claims from what's actually known.

Online forums also push the Macujo method and the Jerry G method, which stack vinegar, salicylic acid acne products, laundry detergent and detox shampoo. They can burn and dry out the scalp, and there's no solid evidence they work. Some versions add bleach. Never combine bleach with vinegar, acids, ammonia or other cleaners, because the mix can release dangerous fumes.

Common myths about the look-back

• "Cutting my hair short resets the clock." The lab cuts at the scalp, so the newest hair is what matters. Short hair just means they may need more of it or switch to body hair.
• "Hair tests only go back 30 days." That's closer to a urine window for heavy marijuana use. Hair is measured in months.
• "They can see exactly which day I used." Even segment testing only gives a rough range of months.
• "Body hair is safer." It's usually the opposite, since it can reach further back.

Tests this page doesn't cover

Don't use detox shampoos or other tricks for DOT, FAA, federal, military, court-ordered, probation or parole testing. Those programs run under strict rules, and any sign of tampering can be treated the same as a refusal. If one of those tests is coming, talk to the program, a medical review officer or an attorney.

Disclaimer and help

This page is general information, not medical or legal advice. Interfering with a drug test may be illegal and can be counted as a refusal to test. Not using is the only guaranteed way to pass. If you'd like support with drug or alcohol use, the SAMHSA National Helpline is free, private and open around the clock at 1-800-662-4357. In a crisis, call or text 988.