

Is this guy a pothead or did he just try it once? Your employer may wonder. A quick urine test will not offer them an answer.

If someone wants to know if you have used drugs recently, they can buy a kit with plastic urine cups and strips that change color like a home pregnancy test. This simple analysis can reveal which drugs someone has used – but not the amount.

Pushing the limits of analytical chemistry, a team of German researchers has found a way to estimate how much weed someone has smoked in the past three months. They described their efforts in a recent issue of Forensic Science International.

Whenever someone uses a drug, a tiny bit of the illicit chemical will become entrapped in keratin – a protein that is the main component of hair. This serves as a rather accurate record of their illicit recreation.

To determine whether that record is precise enough to calculate how much Maryjane someone has smoked, the German chemists took hair samples from 12 male potheads and 10 of their straightedge peers.

The researchers pulverized the hair into a fine powder, dissolved the powder in a strong base, and then extracted all of the volatile chemicals from it with a solvent called hexane. Once the extracts were ready, the researchers used a gas chromatography mass spectrometer to separate out each chemical that was withdrawn from the hair and thus measure the amount of each molecule individually.

By adding up the amounts of tetrahydrocannabinol, cannabinol, and cannabidiol present in a man's hair sample, the researchers could sometimes – but not always – accurately estimate how much weed he smokes. By measuring the amount of THC alone, they could not make an accurate prediction.

Many factors can prevent this test from being completely reliable. If someone gets a lot of sun, ultraviolet light will destroy some of the drug molecules in their hair – lowering the estimate of how much weed they smoke. The same thing will happen if they bleach their hair. Everyone deals with drugs a little differently. Some people pee them out faster than others. Other people have speedy liver enzymes that swiftly destroy drugs. These pharmacogenetic differences also can affect the accuracy of their measurements.