According to a scientific review  , cannabis can typically be detected in saliva for: • Occasional users: 1–3 days • Chronic users: up to 29 days. Blood testing is relatively costly and invasive, making it less common than other testing methods. However, blood tests may be used in some clinical or emergency settings. Unlike urine tests, which detect metabolites, blood tests typically look for the presence of THC itself. THC is almost instantly detectable in the blood upon inhaling cannabis, making blood tests ideal for determining recent drug use.
Research shows  that cannabis remains detectable in blood for 1–2 days. However, in certain cases it has been detected up to 25 days later. Heavy, chronic use of cannabis can increase the length of the detection window. Following consumption of cannabis, THC reaches the hair follicles through small blood vessels. Trace amounts can be detected in hair for up to 90 days, as confirmed by a 2015 study  . Typically, a 4cm section of hair taken from near the scalp indicates cannabis use over a three month period (hair grows at an approximate rate of 1.2–1.5cm per month). If you're required to take a hair follicle drug test, around 100 hairs will be taken from near the crown of your head.
Washing, dyeing, or styling your hair with products will have no effect on the accuracy of the test. So, you've got a drug test coming up and you're feeling uneasy. This may seem obvious, but the first thing you should do if you have a drug test coming up is stop using weed. The aim here is to purge any trace of THC or its metabolites as thoroughly as possible. The longer you cease smoking beforehand, the more likely you are to pass the test. Depending on the type of test and your smoking habits, you may need to take a longer break to fully flush your system. Speaking of flushing your system, there's nothing better than good ol' H₂O. Of course, drinking water is most effective for passing urine tests. To help cleanse your system, pee as much as you can 24–48 hours before your test. The goal is to dilute your urine, making it harder to detect any residual metabolites. Drinking too much water will result in an obviously diluted sample, potentially leading to a retest or increased suspicion. You need to drink plenty of water, but not so much that it becomes obvious. A 2011 study  found that zinc is an effective urinary adulterant, with the potential to produce a false negative test result. Research suggests that oral consumption of zinc supplements after light cannabis use could interfere with detection of THC metabolites in urine for between 12 and 18 hours. Zinc supplements are widely available over the counter in pharmacies and health food stores. So, if a drug test gets sprung on you or you need some extra assurance, you might want to swing by the vitamin aisle. Just make sure to check with your doctor or pharmacist first if you take any regular medications. If you want to kick it up a notch from water, try a detox drink. Detox drinks work on the premise of expelling or masking the presence of THC and its metabolites. Many of them use diuretic substances to make sure you are passing urine frequently. If you don't have the cash, focus on simple detox drinks you can make at home, like coffee or cranberry juice. Another method to prepare your body for a drug test is activated charcoal.
Charcoal is often used to medically manage overdoses; it binds to different substances and helps the body eliminate them. That's because charcoal molecules have a negative electrical charge; they bind with positively charged molecules in different drugs, trapping them. THC metabolites circulate many times through your system via a process known as "enterohepatic recirculation". They move from blood to liver to bile and into the intestines, before being reabsorbed back into the bloodstream. The cycle then repeats, with a portion of the metabolites excreted as waste each time. Eventually, after many cycles, all metabolites are flushed from your system. Research  shows that activated charcoal binds to metabolites like THC-COOH and THC-COO-glucuronide, preventing their reabsorption into the bloodstream and speeding up their elimination. That means charcoal could help you pass a drug test faster.
So, what's the best way to take activated charcoal? Thanks to its purifying properties, charcoal has gained popularity as an addition to cosmetics, health products, and even food. But, let's be honest, it's more fun to do a little baking! The recipe below for charcoal bread results in a unique, black loaf that looks badass and still tastes great. Please note that you should be careful when ingesting activated charcoal—it could interfere with absorption of medications and vitamins.