Even though many public health experts now agree that drug use should not be a punishable offense, the fact is that if you happen to be an Olympic athlete, a government spy, or just the child of ridiculously strict parents, you could be subjected to a drugs test. Failing one of these tests is easier than you might imagine, as many drugs can be detected in a person's urine, blood and hair for some time after the actual effects of these substances have subsided. So even if a few days have passed since a drug was taken, a hell of a lot of biology and chemistry still has to happen before you can give a negative sample.

How drugs get in and out of your system

Once drugs are absorbed into the bloodstream – which can occur via the lungs, the digestive tract, or even a syringe – the only way to get them out is by excretion. Depending on what you’ve taken, some may pass straight through you relatively quickly, coming out in your poop. Much of the rest will eventually be released in your urine and sweat. However, before this can happen, drugs have to be metabolized into water soluble molecules, or metabolites.

This process mostly occurs in the liver, which contains catalysts like cytochrome P450 enzymes that cause drugs to become oxidized. As a result, non-polar molecules – which have no overall charge and are therefore not soluble in water – become negative, much like a drug user on a comedown. Normally, these metabolites will then be ionized as well, ensuring that by the time the liver is through with them, they are well and truly ready to dissolve – just like the ego of someone on LSD.

By this stage, the acute effects of any drug will have worn off, and the soluble metabolites in a user’s system will dissolve into the water in their blood, before being filtered out by the kidneys and excreted as urine. This process can take a while, however, giving drug testers the chance to catch people with drug metabolites in their pee and blood.

Some drugs are excreted faster than others

Because drugs need to be converted into soluble metabolites before they can be excreted, the speed with which this can occur depends largely on what sort of metabolites are created and how concentrated these are. LSD, for instance, is typically taken in very small doses, rarely exceeding 100 to 200 micrograms, which means only tiny amounts of metabolites are likely to be detectable in a user’s system.

Furthermore, most metabolites of LSD tend to pass out of the body very quickly. The main one, for example, is 2-oxo-3-hydroxy LSD, which is completely undetectable in the bloodstream and only present in very small quantities in urine, but only for a few days.

So how long does each drug actually stay in your system for?

The most common types of drug tests will look for drug metabolites in the blood, urine, and hair, the latter of which can become contaminated with traces of drugs as blood washes over the follicle. So without further ado, here’s how long some common illegal drugs remain detectable for:

Cannabis: 7 to 30 days in urine; 90 days in hair; 14 days in blood

Cocaine: 3 to 4 days in urine; 90 days in hair; a day or two in blood

Heroin: 3 to 4 days in urine; 90 days in hair; about half a day in blood

LSD: Up to 3 days in urine; 3 days in hair; less than five hours in blood

MDMA: 3 to 4 days in urine; 90 days in hair; a day or two in blood

Methamphetamine: 3 to 6 days in urine; 90 days in hair; up to three days in blood

Factors affecting the rate of excretion

It’s hard to say exactly how long a given drug will stay in your system, as this will largely be influenced by your rate of metabolism. Obviously, those with a veryhigh metabolism are likely to get rid of drugs much faster than those with a slow metabolism.

Since metabolism tends to slow down with age, older people may be more likely to have drugs in their system for slightly longer than their younger counterparts.
Lifestyle can also have an effect, as those who are physically fit normally have a higher metabolism than those who aren’t.

Additionally, since the excretion of drugs depends on how quickly metabolites can be dissolved, many people assume that by drinking loads of water it is possible to flush them out of their system faster. In theory this could work, although most drug-testing techniques will adjust their results depending on how hydrated the blood is, so they may still catch you out if you go down this route.

How accurate are drug tests?

On the whole, pretty damn accurate. However, studies have revealed that not all techniques are fool-proof, and some can give false results. For instance, one such study found that marijuana smoke can get into the hair of nearby non-smokers, which means these people could test positive for weed even if they haven’t used any. Another study revealed that hair samples from people who have never taken cocaine can become contaminated with cocaine metabolites from users’ hair when being washed in the testing lab.