The very first pharmacological effect of CBD ever observed was related to drug interaction. CBD is both a substrate and an inhibitor of CYP450 enzymes, so it could interfere with the metabolism of other drugs.
(Source: Scientific Reports)
According to a 2018 review, three minutes after smoking a cigarette with 19.2 mg of CBD, the highest plasma concentrations noticed were 110 ng/ml. After an hour of initial administration, the CBD concentrations dropped to 10.2 mg/ml. The average bioavailability of the smoke route is 31%, which means that, if inhaled, CBD tends to get absorbed into the bloodstream at a higher percentage than when taken through the mouth. But, inhaled CBD also clears up faster .
Does CBD Interact With Other Drugs?
When taking CBD through the mouth after a high-fat meal, the absorption of CBD in the intestinal lymphatic system in much higher and shows potential for the treatment of autoimmune diseases. 
The half-life changes depending on the mode of administration and the amount of time CBD is used. Smoking CBD has a 31-hour half-life, and using oral CBD for long periods of time increases the half-life to 2-5 days .
The amount of time CBD remains in the body depends on how much you took, how often you use it, and what form of CBD you’re using.
When applied topically, it is expected for CBD to enter the skin via the transfollicular route and accumulate in the sebaceous gland. This means that CBD can enter through the hair follicles into the sebaceous gland to achieve a local effect .
Research showing how long CBD is detectable in urine is rare, but one study showed a single test subject maintained detectable CBD in his urine for only 24 hours. In any case, CBD is almost never included in standard drug tests (like those given by employers).
Generally, the effects of CBD wear off within just a few hours, but that depends on the amount of CBD taken, the method used, and the user’s size, metabolism, and history of use. If you begin to feel the pain or anxiety begin to return, you can simply take more CBD. There is no practical way to overdose on CBD.
CBD disappears from the body relatively quickly. In a study following 14 patients who were each given about 700 mg of CBD a day orally for six weeks, the average plasma level fell to just 1.5 ng/mL a week after the final dose, and was undetectable after that. The authors estimated the CBD half-life to be about 2-5 days.
How long does CBD keep working?
Because inhaled CBD reaches the brain rapidly, users may feel a much more immediate and pronounced effect than they would from tinctures or edibles. But the therapeutic effects—reduced inflammation, pain and anxiety—are eventually noticeable from all forms of consumption, assuming you use enough. Proper CBD dosing can require some experimentation.
The non-intoxicating compound CBD is known to have a variety of beneficial effects, including reduction of pain and inflammation, and anxiety relief. A single dose of CBD may keep working for several hours, and the cannabinoid may persist in the body for days or weeks. How long CBD effects last depends on several factors, including the user’s metabolism and if they use CBD regularly.
CBD (or cannabidiol) is stored in fat cells in your body, and theoretically can remain detectable for days or even weeks in a blood test. However, research on how long CBD oil stays in the system is spotty, because there’s no real demand for testing. Few employers care if employees consume a non-intoxicating drug that is used mainly to treat pain and anxiety—and commercial testing protocols don’t measure CBD anyway.
It generally takes 5.5 half-lives for a drug to be fully eliminated from the system. So CBD taken orally would completely leave the body in about 11-28 days, but it may be impossible to detect on a blood test well before that. The half-life for vaped or smoked CBD in blood plasma is much shorter than for ingested CBD—just a few hours. A regular user may take longer to fully clear CBD from their system than an occasional or new user.