How long does THC stay in hair?

What the test measures

Cannabinoids reach the hair as it grows, so a length of hair is a record of the weeks it grew. In the study cited here, hair was cut into 1 cm sections, each standing for about one month of exposure, and tested for five cannabinoids, THC among them.^[9]

Typical windows

How often	Reported window
Once, or now and then	Often not detected[9]
Several times a week	Not well studied
Daily	About the last 3 months[9]
Heavy daily use for months	About the last 3 months[9]

What the study found

Researchers tested hair from 136 people who described themselves as heavy, light or non-users of cannabis.^[9]

Group	Tested positive for THC
Heavy users	77%[9]
Light users	39%[9]
Non-users	0%[9]

The amount of THC found correlated only moderately with the amount people said they used each month (r = 0.52).^[9] The authors concluded that hair works as a qualitative sign of heavy, daily or near-daily use within the past 3 months, but cannot reliably detect light use or measure how much someone used.^[9]

What a hair test cannot tell you

• Not a date. Each centimetre stands for about a month, so a result describes a period of weeks, not a day.^[9]
• Not a single occasion. Light use was missed more often than it was found.^[9]
• Not a dose. The correlation with reported use was too loose to work out quantity.^[9]

Before you rely on a number

This page rests on one study, and hair tests vary between labs. A range from a study is not a prediction for one person; only a lab test can tell you your result.