How Long Does Marijuana Stay in Your Hair?

As soon as someone finds out that they have to take a hair follicle drug test, one of the first questions they’re to ask is “How long does marijuana stay in your hair?”

If one looks around on the internet they may find answers ranging from 90 days to 90 years.

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So how long does marijuana stay in your hair?

If you’re being asked to take a hair follicle drug test, you only have to worry about the last 90 days.

With hair follicle drug testing, the lab that takes the sample will cut a small batch of hair from as close to the scalp as possible near the crown of your head.

They will then trim that down to 1.5 inches so they are testing the most recent 90 days of growth.

This is based off the assumption that hair, generally, grows at an average length of 0.5 inches per month. So 0.5 inches x 3 months (90 days) = 1.5 inches.

But is that how long marijuana stays in your hair?

No. Not really.

First off, we’ve got a 7 – 10 day window that we need to account for. When drugs enter your hair follicle, it enters below the scalp level. It takes 7 – 10 days for the portion of hair that contains marijuana or other drug metabolites to push through the scalp.

So, in essence, if the lab takes a 1.5 inch sample, they’re testing the last 90 days but with a 7 – 10 day fudge factor built in.

For example, assume that on Jan 1, I ingest some marijuana. So, in theory, if you were to be tested anytime after April 2 (90 days late), you would be good to go.

But, the hair that contains drug metabolites did not push up through the scalp until around Jan 10. That means if the drug test lab takes a 1.5 inch sample on April 2, they’re really testing any hair that has exited the scalp between Jan 10 and April 2.

The upside of that is that if you never smoked marijuana before and you went to a concert last night and took a few hits, and were given a surprise hair follicle drug test the next day, you would be highly likely to pass because no hair that contains drug metabolites has pushed through the scalp line yet.

However, this is not an exact science. Hair does not grow at a constant rate. In other words, my hair may grow slightly slower or faster than your hair. That could be based on anything ranging from genetics to diet. The 0.5 inch per month rate is simply the average.

Further muddying the waters is the fact that your hair may not grow at the same rate on different parts of your body.

Your underarm hair may grow at one rate and the hair on your head a completely different rate. The 0.5 inch per month average is for hair growing on your head.

If you are naturally bald or have chosen to shave your head, 1.5 inches of underarm hair may give a sample that gives a picture much longer than 90 days.

So how long does marijuana stay in your hair, really?

Well, first off, anybody that tells you that it stays in your hair forever is sorely mistaken. First off, every time you cut your hair you’re cutting off a piece of history.

Secondly, the drug testing labs test for the presence of drug metabolites, byproducts of your body metabolizing the drug, which can become unstable over time and break down into other chemical compounds that the drug testing labs are not testing for.

So, if you’re 30 years old and you smoked weed 10 years ago, unless you haven’t cut your hair in 10 years, you’re not going to have any hair sections on your head that were around 10 years ago. And even if you did, there’s a good chance that the drug metabolites in that hair would have become unstable and broken down into other compounds that are not being tested for.

This is why the drug testing labs cut the hair down to 1.5 inches. They know that anything beyond 90 – 100 days is likely to have degraded to the point where they can’t scientifically establish a timeframe with any reliability.

So when you consider “How Long Does Marijuana Stay in Your Hair?” you really need to be focused on what they are testing for. The drug testing lab is going to be testing for roughly the last 90 – 100 days since they are only testing a 1.5 inch sample.

If they are taking hair from other parts of your body, that window may be much longer. How much longer? It’s difficult to predict without measuring the exact growth rate of your particular hair for that specific part of your body.

But, it’s certainly not forever.