Cannabis stays in your system for around 30 days and can be detected in your bodily fluids during this period. It can stay in your hair for even longer: hair tests can detect cannabis use for up to 90 days.
But whether cannabis use shows up on a test depends on how much cannabis you used, and over what length of time. Drug tests are more likely to pick up on heavy and frequent use.
Working out how long cannabis stays in your system, and whether it will be picked up by a drug test, is not an exact science.
In this article, we go through some of the factors which determine the length of time cannabis stays in your system. We also discuss different drug tests, how accurate they are, and how likely they are to pick up cannabis use.
What are the main drug tests, and how good are they at detecting cannabis use?
The main drug tests include saliva tests, urine tests, hair tests, and blood tests. These tests measure tetrahydrocannabinol, or THC, as well as other cannabinoids.
Cannabis enters the bloodstream very quickly, especially when it is smoked (with edible cannabis it may take longer to do so). It is then metabolised, and the by-products of this reaction can stay in the bloodstream for a while.
One of the most common forms of drug test is the urine test. This is a popular form of test because it is relatively easy to carry out, does not need to be done in a laboratory setting, and is still quite accurate.
Urine tests can detect cannabis use for up to 30 days, although this figure corresponds to the heaviest and most frequent use (several times a day).
With infrequent use (up to three times a week), a urine test will only detect cannabis for up to 3 days. More common use, when the user is smoking cannabis around four times a week, can be detected by a urine test 5 to 7 days after the event.
Chronic or daily use can be detected for 10 to 15 days by a urine test.
Saliva tests are less common than urine tests, but may still be used because they offer a lower ‘adulteration potential, ease of multiple sample collections, lower biohazard risk during collection’ among other advantages. 
Cannabis metabolites can be detected in oral fluid for up to 29 days, in chronic users. For infrequent users, saliva tests can detect cannabis 1 to 3 days after use.
Oral fluid tests work by measuring cannabinoids such as THC (tetrahydrocannabinol) in the saliva. These cannabinoids enter the saliva after cannabis is smoked, inhaled, or sprayed into the mouth.
Hair tests are by far the most accurate form of drug test and can detect cannabis for up to 90 days after use.
Hair tests work because cannabis (and other drugs) leave trace amounts in the hair follicles after use.
Since hair grows at a rate of half an inch per month, scientists can measure what drugs a person has taken by looking at a small segment (1 and a half inches) of hair from close to the scalp.
Hair tests do come with some disadvantages, however. The biggest disadvantage is false positives. False positives in drug tests are when a test shows up as positive, despite the person being tested not having used cannabis.
This can happen with hair tests because they measure oil in the hair, and this can be contaminated (e.g. if the person being tested had come into contact with someone who used cannabis).
If you are tested for cannabis at work, you will likely be given an immunoassay test. This test, also known as the EMIT test, is not 100% accurate; if the test comes back positive you will be given a more accurate test such as the gas chromatograph-mass spectrometer test (GCMS).
By having two rounds of testing, the chances of false positives influencing the result are lower.
In general, drug testing has got more accurate over time, meaning that false positives are less of a problem than they were in the past. Ibuprofen used to cause false positives in cannabis tests, but that is no longer an issue with today’s testing.
The psychoactive component of cannabis is called THC, or tetrahydrocannabinol.
When you use cannabis, this chemical is absorbed into the blood. It then passes to the organs and fatty tissues before eventually being metabolised in the liver.
THC has over 80 metabolites. Even after the THC has left your bloodstream, these metabolites remain. Drug tests try to pick up these metabolites, either in the hair, blood, urine, or saliva.
What other factors determine the length of time cannabis stays in your system?
As well as the extent and frequency of cannabis use, there are several other factors that can affect how long cannabis (and THC) remain in your body.
Weight is of particular importance since the body stores THC in fatty tissue.
A person with more fat (relative to their height, age, sex etc.) may find that cannabis stays in their system for longer. By contrast, someone who has a lower BMI (Body Mass Index) and exercises regularly may find that marijuana does not remain in their system for as long.
The potency of cannabis can also play a part. Stronger cannabis is likely to remain in your system for a longer period of time since it will contain a greater quantity of THC.
Edible cannabis may also stay in your system for longer than smoked cannabis, since it takes longer to break down than if smoked.
If there are other drugs in your system, this can also have an impact on the speed at which THC is metabolised. There are several drugs that increase the amount of THC in your body, among them verapamil, itraconazole, erythromycin, and clarithromycin.
Rifampin has the opposite effect, decreasing the levels of THC in the body.
Although you can find lots of remedies on the internet for getting cannabis out of your system, none of these are backed up by scientific evidence.
Once you have used cannabis, your only real option is to let your body get to work. If you have a drug test that you need to pass, then all you can do is hope that the cannabis leaves your system in time.
That being said, we do not recommend using cannabis in the first place, as it has been linked to mental health disorders such as bipolar disorder. 
What are the effects of using cannabis, and how long do they take to appear?
The effects of cannabis don’t take long to appear, especially when the drug is smoked. You can expect to feel a buzz after as little as 15 minutes.
When ingested, the effects of cannabis take longer to appear: between one to two hours.
The effects of cannabis use include:
Further effects can include:
Smoking cannabis has also been known to cause hallucinations and psychosis in some people, although this is comparatively rare.
Long-term use of marijuana can cause developmental issues such as memory impairment and cognitive impairment. It can also cause diseases such as bronchitis and heart disease and puts you at greater risk of a stroke.
Finally, smoking cannabis is not recommended during pregnancy as it can negatively impact your unborn child.
Effects such as giggling, clumsiness, drowsiness, and so on will tend to wear off after 1 to 3 hours.
However, more long-term effects such as insomnia can last longer, depending on the frequency of use.
The length of time it takes for cannabis to be metabolised varies greatly from person to person, depending on age, weight, quantity of cannabis used, method of use, and so on.
On average, cannabis stays in your system for between 10 and 30 days.