The question “how long does weed stay in your hair?” is now an increasing cause for concern among many people who take cannabis for medical or recreational purposes. Law enforcement officials, conservative employers, and insurers these days are slowly moving towards hair follicle testing as a primary detection method for cannabis use. This method of testing has a detection window approximately three times that of traditional urine testing, which remains the standard THC testing method today.
The hair follicle testing method can detect cannabis use approximately 90 days from the last time a user consumed a THC product and can even be used to screen for other types of substances including amphetamines, cocaine, PCP, opioids, and others.
Washing, coloring, or otherwise treating your hair will also do nothing to reduce the detectability of THC in hair. This makes it a particularly difficult test to evade compared to traditional urine tests.
How hair follicle testing works
Testers will take between 100 and 200 individual hair follicles from your head or other parts of your body, as may be needed if you’re bald. These hairs will then be processed at a laboratory where the testers will screen for THC or any of a number of different illicit drugs. Testers will typically pay close attention to the roots of the hairs where they meet the scalp. If there is present sebum, this could be processed and checked for any signs of drug use. Typically at least two tests are done in order to guard against false positives and increase the validity of any findings.
There are also several hair testing methods, and they may be repeated or combined as the tester wishes to further reduce the possibility of false positives. This makes getting a negative result if you had recent drug use incredibly difficult.
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Issues with hair follicle testing
Hair follicle testing is not perfect by any means. Hair follicle testing has been around since the 1980s, but adoption has been slow for several reasons.
The first reason is false positives. Accidental drug ingestion does happen, and these tests do not prove beyond a reasonable doubt that a person is abusing cannabis or any other drugs. In a landmark case involving police officers, many tested positive for cocaine in a hair test. However, the court ruled that the hair test did not prove beyond a reasonable doubt that the police officers did use cocaine. The same can be true for any hair testing case that results in positive THC findings.
The second issue is that different people metabolize THC at different rates and in different ways, which means you may take cannabis together with someone and they may pass the hair test while you fail it.
Another issue is that hair with more melanin is more likely to retain THC and other drug metabolites, which can be problematic for many darker-haired people and many racial minorities as well.
Perhaps the most compelling problem is the high cost of hair testing. Hair testing kits can cost anywhere from $64 to $125, compared with $30 to $60 for a standard pee test.
Conclusion
The answer to “how long does weed stay in your hair?” is far more complex than you might expect. “While hair tests can detect cannabis use three months from the last time it was consumed, the test is by no means perfect. If you are required to take a hair test, try to see if you can test yourself first with a home kit, and try to bide your time or delay testing if possible. Or you may want to seek employment somewhere that does not persecute the use of medical or recreational cannabis.