Alcohol can be detected in hair for up to 90 days using a standard test, and potentially longer if a longer sample is collected. The 90-day window comes from a simple calculation: head hair grows about 1 centimeter per month, and the standard sample is 3 centimeters (roughly 1.5 inches) taken from the scalp. Each centimeter represents approximately one month of history, giving labs a three-month lookback.
What the Test Actually Measures
Hair alcohol tests don’t detect alcohol itself. Alcohol leaves your blood within hours. Instead, labs look for a byproduct called EtG (ethyl glucuronide), a stable compound your liver produces when it processes ethanol. As your blood circulates past hair follicles, EtG gets incorporated into the growing hair shaft and locked into the keratin structure. Once embedded, it stays there essentially as long as the hair exists.
The amount of EtG in your hair depends on two things: how much alcohol you consumed during that growth period, and how efficiently your body metabolized it. Individual metabolism varies, which means two people drinking the same amount won’t necessarily show identical EtG levels. Conditions like liver disease, kidney dysfunction, and diabetes can also affect how much EtG your body produces and how much ends up stored in hair.
Why 90 Days Is the Standard
Labs typically collect a pencil-thick strand of hair, cut as close to the scalp as possible, and test the 3 centimeters nearest the root. Since hair grows at roughly 1 centimeter per month, that 3-centimeter section represents the most recent three months of drinking history. Hair farther from the scalp is older, so a longer sample can theoretically extend the detection window well beyond 90 days.
There’s a practical reason labs stick to 3 centimeters, though. The farther you get from the scalp, the more that section of hair has been exposed to washing, sunlight, and environmental wear. These factors can gradually degrade the markers labs are looking for, making results from older sections less reliable.
Segmental Analysis: A Month-by-Month Timeline
In some legal and clinical settings, labs don’t just test the full 3-centimeter sample as one unit. They cut it into individual 1-centimeter sections and test each one separately. This is called segmental analysis, and it creates a month-by-month profile of your drinking patterns. The section closest to your scalp reflects the most recent month, the next section reflects the month before that, and so on.
This technique is commonly used in custody disputes, DUI cases, and professional licensing reviews where the question isn’t just “did this person drink?” but “when did they drink, and have they stopped?” If someone claims they quit drinking four months ago, a segmental analysis of a 6-centimeter sample could confirm or contradict that by showing whether the older sections test positive while the more recent ones come back clean.
How Hair Tests Compare to Other Methods
The 90-day window for hair dwarfs every other testing method. A standard urine EtG test detects alcohol use for roughly 48 to 80 hours after drinking. Blood alcohol tests are even shorter, typically useful for only about 12 hours. A blood test called PEth can detect heavy drinking over the past three to four weeks, which makes it the closest competitor to hair, but it still covers less than half the window.
The tradeoff is that hair testing can’t tell you whether someone had a drink yesterday. It takes about 7 to 14 days for a newly grown section of hair to push above the scalp where it can be collected. So hair tests have a blind spot for very recent use. That’s why they’re often paired with a urine test in monitoring programs: urine catches the last few days, hair catches the last few months.
What Can Affect Your Results
Chemical hair treatments can significantly alter EtG levels. Research published in Therapeutic Drug Monitoring tested the effects of common salon treatments and found stark differences. Regular hair coloring (dyeing without bleach) had no meaningful impact on EtG concentrations. Bleaching, however, reduced EtG levels by an average of 73.5%. The most dramatic effect came from perming, which destroyed an average of 95.7% of the EtG in tested samples.
Bleaching appears to work through a combination of chemically breaking down EtG and physically stripping it from the hair shaft. Hydrogen peroxide, the active ingredient in bleach, degraded nearly half of EtG on its own in lab experiments. Perming chemicals (specifically ammonium thioglycolate) were even more destructive, causing near-total chemical breakdown of the marker.
This doesn’t mean bleaching your hair guarantees a negative result. Labs are aware of these effects and may note signs of chemical treatment. Some testing programs use a second marker called FAEE (fatty acid ethyl esters) alongside EtG, partly because the two markers respond differently to cosmetic treatments. If one marker comes back suspiciously low while the other doesn’t, that discrepancy can itself raise questions.
What About Body Hair?
When head hair isn’t available (due to shaving or baldness), labs can collect body hair from the chest, arms, or legs. Body hair grows more slowly than head hair and has a different growth cycle, which complicates the timeline. Rather than representing a neat month-per-centimeter calculation, body hair samples generally reflect an approximate window of several months to a year, but without the precision of segmental head hair analysis. The results still indicate whether someone consumed alcohol during the growth period, but pinpointing exactly when becomes much harder.
What Hair Tests Can and Can’t Tell You
Hair testing is designed to identify patterns of repeated drinking, not single isolated episodes. The Society of Hair Testing (SoHT) has established threshold levels that labs use to distinguish between chronic heavy drinking and lower levels of consumption. Results below a certain concentration suggest either abstinence or very light, infrequent drinking. Results above a higher threshold point to chronic excessive use. The space between those cutoffs is where interpretation gets nuanced and context matters.
A single night of heavy drinking may or may not produce enough EtG to trigger a positive result, depending on your metabolism and how much you consumed. Several drinks spread over multiple occasions during a month are more likely to register. The test is most reliable as a tool for detecting sustained, repeated alcohol use over weeks and months, which is exactly why it’s favored in legal and clinical monitoring programs where the question is about ongoing behavior rather than a one-time event.