How long is hair detection window?

Hair drug testing can often detect substance use for weeks to months depending on the length of hair analyzed and the substances being tested. Standard hair testing commonly evaluates approximately 90 days of exposure because laboratories typically analyze the most recent 1.5 inches of scalp hair growth. Longer hair samples may extend the detection window further in some testing situations.

Hair testing works by identifying drugs or metabolites incorporated into the hair shaft as hair grows. Once substances enter the bloodstream, small amounts may become embedded within developing hair follicles and remain present as the hair continues to grow outward. Because hair grows gradually over time, testing reflects a historical pattern of exposure rather than immediate intoxication.

The exact detection period depends on hair growth rate and laboratory collection procedures. Human scalp hair generally grows about half an inch per month, although growth speed varies between individuals. Body hair may produce less predictable detection timelines because growth cycles differ from scalp hair.

Hair testing is generally more useful for identifying repeated or long-term substance exposure than very recent use. Drugs consumed within the previous several days may not yet appear in newly formed hair above the scalp surface. Other testing methods such as blood, saliva, or urine are typically more effective for detecting recent exposure.

Laboratory methodology and testing sensitivity also influence the effective detection window. Different substances incorporate into hair at different rates, and cutoff thresholds vary between testing programs. Interpretation of hair test results therefore commonly considers hair length, growth patterns, testing procedures, and the specific substances being evaluated.