How far back can a hair drug test detect drug use?

A hair drug test typically detects drug use over the past 90 days, though the exact window depends on hair length and growth rate. Hair testing is designed to show patterns of use over time rather than recent or one-time consumption.

As hair grows, drug compounds and their metabolites are incorporated into the hair shaft from the bloodstream. Standard testing usually analyzes the first 1.5 inches of hair closest to the scalp, which corresponds to roughly three months of growth for most people. Longer hair can extend the detectable period further back if additional segments are tested.

Hair tests do not reliably detect very recent use. Because it takes time for hair to grow above the scalp, drug use within the past 5 to 7 days may not appear in results. For this reason, hair testing is often used alongside urine or blood testing when recent use needs to be assessed.

Detection sensitivity varies by substance. Cocaine, amphetamines, opioids, and PCP are commonly detected through hair testing. Cannabis can also be detected, though results may be less consistent for infrequent use. Cosmetic treatments such as bleaching or heavy dyeing can reduce drug concentrations but do not reliably eliminate detection.

Understanding hair testing helps clarify its purpose. It is not a snapshot of current intoxication, but a long-term record of exposure. Results reflect repeated or ongoing use over weeks to months rather than isolated or very recent events.