“How long does weed stay in your hair?” – It may seem like a trivial enough question but it’s important to know when you consider that hair tests are often employed for detecting marijuana use.
Although the majority of states have legalized marijuana in varying forms and degrees (with many more scheduled to follow suit) there are still many states in the U.S. that allow employers to implement drug testing procedures. Given this factor, knowledge about how long marijuana remains in the hair can be very helpful.
It is important to make a distinction between the perceivable mental and physical effects of cannabis and how long the drug remains detectable in the system. Although the effects of marijuana generally fade after a few hours, the primary active ingredient, tetrahydrocannabinol (THC), can be detected in the body for up to several weeks via any one of the many drug-testing procedures commonly implemented by employers and government agencies.
Dependent on multiple factors
The length of time during which THC can be detected in the system is dependent on factors such as its metabolic rate and half-life, the user’s dosage and frequency of use, and body-fat percentage. In general, the more frequently a person smokes cannabis, and the more he consumes per session, the longer the period of time in which cannabis will be detectable in his system.
A number of THC metabolites have a half-life of about 20 hours, which means that it will take that long before they are completely eliminated from the system. Other compounds have much longer half-lives, remaining in the body’s fat cells for up to 10 or 13 days. The half-life of THC itself is dependent on the user’s weight, and the length, frequency, and volume of use.
Commonly administered drug test
So how long does weed stay in your hair? For most people, THC is detectable in their hair for anywhere from 7 to 10 days from the time of last cannabis use. At the end of this time, the section of the hair typically examined during a drug test will have grown out sufficiently. In most tests, the newest growth of hair measuring 1.5 inches is examined. It usually takes 90 days after last use for the entire hair strand to be entirely free of THC.
Hair tests are among the most commonly implemented testing procedures for determining the presence of THC. They are preferred by some testers due to the ease in which hair can be collected, stored, and transported. Other tests involve the examination of blood, saliva, or urine. Hair tests are typically administered as part of a comprehensive drug testing program.
Hair tests for marijuana involve the examination of the hair shaft rather than the entire hair strand. This reduces the likelihood of contamination. Some testers may opt to collect body hair instead, which grows at a much slower rate than head hair, and will therefore retain traces of THC for much longer periods. The presence of THC in the hair cannot be masked, diluted, or eliminated by using shampoos or bleaches, or by flushing the system out with water.