Hair follicle drug tests typically detect drug use going back approximately 90 days (about three months), based on the standard 1.5-inch hair sample taken closest to the scalp. This length corresponds to the average hair growth rate of about half an inch per month, allowing the test to provide a retrospective window of drug consumption over that period

. However, hair drug testing can technically go back much further if longer hair samples are available. For example, a 12 cm hair sample (about 4.7 inches) can represent up to 12 months of drug use history, but this requires segmented analysis-dividing the hair into consecutive 3 cm sections to maintain accuracy and provide a timeline of drug use over the year. This extended testing is more complex, costly, and less commonly performed but is used in legal or forensic contexts where long-term drug history is needed

. Body hair can also be tested if scalp hair is unavailable, potentially extending the detection window up to a year or more, though the timing is less precise due to slower and variable growth rates

. In summary:

• Standard hair follicle tests detect drug use up to 90 days back using 1.5 inches of scalp hair.
• Extended testing with longer hair samples and segmented analysis can detect drug use up to 12 months.
• Body hair testing may extend detection further but with less precise timing.

These tests are more reliable than urine tests for detecting drug use over longer periods and are widely used in employment screening, legal cases, and other settings requiring drug use history