How long is THC detectable in hair?
Hair tests detect THC use over the hair growth period — typically about 3 months per 3 cm of hair. Longer hair or segmental analysis can reconstruct 6–12 months of consumption history. Sensitivity depends on amount and frequency.
Key facts
- ~3 months per 3 cm of hair
- Segmental analysis: up to 12 months
- Regular use: reliable
- Dyeing lowers values, not detection
Detailed answer
Hair analyses (SoHT guidelines) measure THC metabolites like THC-COOH, incorporated into growing hair via the bloodstream. At ~1 cm/month hair growth, a 3 cm segment corresponds to 3 months. Regular use is reliably detectable; single use can stay below the cutoff. Cosmetic treatments (dyeing, straightening) lower values but don't fully prevent detection.