Urine tests can detect the THC in the fat cells, since some of it gets washed out with the urine. THC can reside in fat cells for up to 4 weeks, sometimes longer if the person consumes large amounts of marijuana.
In theory, the same reason you can’t get high from rubbing them on your skin related is to why using topicals will not cause you to test positive in a drug test.
Whether you support the idea of drug testing or not, the consequences of having a positive test can be far reaching and detrimental.
In reality, the only thing that will ensure a clean drug test is abstaining from ingesting drugs, at least for a while, but each test is different in terms of how long you need to abstain and what the test can tell about your use history.
Amanda Reiman, PhD.
Topicals are infused with a variety of cannabinoids. The most common is CBD, which does not contain any of the psychoactive properties that cannabis is renowned for. Instead, it is utilised for its medicinal benefits. However, some topicals do contain other cannabinoids like THC and CBN. Given that THC is the key psychoactive component in cannabis that enables users to get high, it is understandable that some remain sceptical about the use of topicals.
Cannabis topicals are an incredibly useful way to get localised pain relief and reduce inflammation. With the market expanding rapidly, their use is becoming significantly more widespread, even extending outside of traditional cannabis users.
With topicals unable to get users high, it would stand to reason that it’s not possible to fail a drug screening as a result of their use. In principle, this is true; but there are some caveats. This study from 2017  confirmed that topicals containing THC did not cause a positive test in both blood and urine.
WILL TOPICALS GET YOU HIGH?
With a localised application, sufferers can target specific areas of trouble, supporting mild pain relief and reducing inflammation. It is the nature in which they are absorbed through the skin, however, that has raised questions about their safety. Will users get high as a result? And furthermore, would that cause a positive reading on a drug test?
The caveat is transdermal patches. These work in the same way as a nicotine patch, providing patients with a strong dose of the active ingredient that is absorbed into the bloodstream through the skin. As such, use of THC-rich cannabis patches is likely to result in a failed drug test.
The thing is, our skin acts as a protective barrier, which it does an excellent job at. To give you an example of how topicals cannot get you high, consider rubbing alcohol. Those who apply alcohol to a cut are still able to drive afterwards without fear of being over the legal limit. This is because it cannot be absorbed into the bloodstream through their skin. Cannabis topicals work on the same principle. The cannabinoids bind to receptors in our skin, muscle tissue, and nerves, but get absorbed before they could permeate through our skin into the bloodstream. THC only gets users high when it reaches our brains, something it cannot do through the surface of the skin.
“Topical” is a term used to describe products applied to the surface of the body. Balms, ointments, lotions, and salves are all included in this category. What makes them unique is that rather than being consumed orally, they are applied directly to the skin. For that reason, they are not taken as a recreational drug, but instead for exclusively therapeutic use.