Marijuana is Lipophilic - this means fat-soluble.

A person gets high from marijuana when THC travels via their bloodstream to the brain and is processed through the endo-cannabinoid receptor system. Almost immediately and while the person is still very much high, the body starts to pull the THC out of the bloodstream and deposit it in fatty tissue around the body. The brain happens to be a large piece of fat, so quite a bit is stored there.

The body does this with most fat-soluble toxins floating/encountered in the bloodstream.

After the THC stops entering the blood (i.e. the person stops using) and most of it has been cleared (pulled into the fatty tissue), and usually after a person stops being high, the body starts to slowly leach the THC from the fatty tissue back into the bloodstream. At these low-levels, the THC is carried by the blood to the body’s excretory systems, i.e. breathing, sweat, urination, etc… for elimination from the body. This form of THC is usually referred to as a metabolite.

Again, the body does this with most fat-soluble toxins stored in fatty tissues.

It can take 2-4 weeks to fully expel all THC from the body, and if more THC is consumed in the meantime, there is cumulative THC build-up that the body is continuously trying to evacuate.

This is why we hear people say that they, “tested positive for THC weeks after they smoked or consumed marijuana.” These metabolites are being picked up in blood or urine tests.

Lastly, while THC is being evacuated from the body, this metabolite-type THC may be affecting the user in a low-grade way that they don’t consciously detect. The affect is typically minor, and most users simply can elevate above it - not feeling a “high” effect. But we know these low levels can be detected by venal (blood-draw) and urine drug tests. They can also be detected by sensitive physical tests designed to evaluate coordination and memory.

A note on Alcohol: Alcohol is water soluble and is typically out of the body within 3-24 hours of consumption end (dictated by a person’s weight, metabolism and amount consumed). Alcohol leaves the bloodstream at a steady rate that is consistent among all drinkers when these components are factored in. This allows for levels of intoxication to be consistently identified and is the basis for the BAC (Blood Alcohol Content) marker system. There are also physical field-sobriety tests that can detect alcohol impairment. 

Similarly, there are some specific field-tests that can identify marijuana impairment vs other kinds, but fluid-based tests that consistently tie levels of THC/Marijuana to impairment are not available yet.