Substance guides

Hair follicle drug test: What it detects and for how long

Published April 28, 2025 · 6 min read · Updated April 2026

Reviewed for accuracy by licensed clinical professionals.

Hair follicle testing provides the longest detection window of any drug test method, detecting drug use patterns over approximately 90 days.

How it works

Drug metabolites are incorporated into hair as it grows. A standard sample (1.5 inches of hair closest to the scalp) represents approximately 90 days of growth. The sample is washed to remove external contamination, then analyzed.

What it detects

All standard drug classes. Detects patterns of repeated use more reliably than single-use episodes. Very low-level single use may not be detected.

Limitations

Cannot determine exact timing of use within the 90-day window. May show racial bias (some studies suggest drugs bind more readily to darker, coarser hair). Body hair can be used if head hair is unavailable but represents a longer, less precise window. Very short hair may provide a shorter detection window.

Authoritative sources

This article references guidelines from: NIDA · SAMHSA · CDC

Frequently asked questions

How far back does a hair drug test go?

Standard testing covers approximately 90 days. Longer hair can theoretically detect further back but standard protocol tests 1.5 inches.

Can you beat a hair follicle test?

Shampoo products marketed as detox are unreliable. Shaving all hair is suspicious. The most reliable way to pass is abstinence for 90+ days.

Does one-time use show on hair?

Single low-level use may not reach detection thresholds. Hair tests are better at detecting repeated use patterns.

Disclaimer: Informational only. Not medical advice. SAMHSA: 1-800-662-4357.