Hair drug testing can provide a longer retrospective view of selected drug exposure than urine, blood, or oral fluid. It does not create an exact calendar of use, and it is generally less useful for detecting very recent exposure. Interpretation requires attention to sample location, segment length, growth patterns, cosmetic treatment, contamination controls, analytes, and laboratory methods.
Compare hair with other specimens in Drug Testing Methods.
How drugs enter hair
Drugs and metabolites may become incorporated into hair through blood during growth, sweat and sebum, and external exposure. Laboratories use washing procedures, analyte selection, metabolite patterns, and confirmation criteria to help distinguish ingestion from contamination, but interpretation is not always simple.
How hair is collected
A collector generally cuts a small sample close to the scalp, commonly from the crown area. The root end is identified so the laboratory knows the direction of growth. When scalp hair is unavailable, a policy may permit body hair, but body-hair growth patterns differ and should not be interpreted as equivalent to a scalp segment.
The amount, length, collection site, chain of custody, packaging, and cosmetic condition should be documented. Collection is usually directly observed and does not require a restroom.
What period does hair represent?
Scalp hair grows at an average rate often approximated at about one centimeter per month, but individual growth varies. Laboratories may analyze a defined segment near the scalp to represent an approximate retrospective period. That approximation is not a day-by-day timeline.
Hair must grow beyond the scalp before it can be collected. Processing and incorporation also take time. Hair testing is therefore generally not the preferred method for identifying exposure in the immediately preceding days.
What can be tested?
Panels may include cocaine-related analytes, opioids, amphetamines, methamphetamine, cannabis-related analytes, PCP, and other substances. The panel and laboratory method must be verified. Hair is not currently an authorized specimen for federal agency workplace testing under the HHS Mandatory Guidelines.
Laboratory process
- The sample is received, identified, and accessioned.
- The relevant segment is measured and prepared.
- External material is addressed through validated washing procedures.
- The hair is cut, pulverized, or otherwise processed.
- An initial screen may be performed.
- Non-negative findings are confirmed with a specific analytical method.
- Results are interpreted according to the program and laboratory criteria.
Advantages
- Longer retrospective view than many other specimens.
- Observed collection with low substitution risk.
- Specimen is relatively stable and easy to transport.
- Segmental analysis may provide limited information about different historical periods when scientifically appropriate.
Limitations and interpretation issues
- Recent exposure: very recent use may not yet be represented in collectable hair.
- External contamination: environmental exposure must be addressed through laboratory procedures and interpretation.
- Cosmetic treatment: bleaching, dyeing, straightening, and other treatment can affect analyte recovery.
- Hair characteristics: color, melanin, growth, and structure may influence incorporation and raise fairness considerations.
- Body hair: it may represent a less precise and potentially longer period than a defined scalp segment.
- Timing: a result cannot identify an exact day of use.
- Impairment: hair does not establish current impairment.
Positive and negative results
A confirmed positive means the specified analyte met the laboratory and program criteria in the tested hair segment. It does not by itself establish dose, frequency, exact timing, impairment, or a substance-use disorder. A negative result means the included analytes did not meet reporting criteria; it does not rule out exposure outside the represented period, below the cutoff, or to a substance not included in the panel.
When hair testing may be appropriate
Hair may be considered when a longer historical pattern is relevant and the governing program permits it. The decision should account for scientific limitations, legal requirements, fairness, confirmation, review, and dispute procedures. It should not be selected merely because it is described as “harder to beat.”
Frequently asked questions
Can shaving prevent hair testing?
Collection policies may permit body hair or another specimen when scalp hair is unavailable. Deliberate removal can also have consequences under a testing policy. This site does not provide advice for evading collection.
Can a laboratory tell exactly when a drug was used?
No. Segment length can provide an approximate historical framework, but growth variation and incorporation prevent exact dating.
Does secondhand exposure cause a positive?
Laboratories use washing, confirmation, metabolite patterns, and cutoffs to address contamination. A result should be evaluated under the laboratory’s validated criteria rather than assumed from exposure alone.
Related guides
Sources
Last evidence review: October 29, 2026.