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Dr Ian Nnatu

Page medically reviewed by Dr Ian Nnatu (MB BS, PG DIP (CBT), MSc, FRCPsych, MRCPsych) Consultant Adult Psychiatrist at Priory Hospital North London

Marijuana, also known as weed, cannabis or pot, is a drug that is derived from the cannabis sativa plant. Within the plant is a chemical known as tetrahydrocannabinol (THC), which produces a ‘high’ when someone smokes, ingests or vapes it.

UK statistics show that almost 30% of people aged 16-59 have taken marijuana at least once in their lifetime. THC can result in feelings of calmness, relaxation and tranquillity, which can make people want to take this drug over and over again. However, repeated use can result in you developing a harmful addiction to this substance.

Weed might be detected in your system for up to 90 days after you’ve taken the drug, depending on a number of factors, including:

  • The method of testing for cannabis in your system
  • The way in which you consumed the cannabis (e.g. smoking or ingesting the substance when it has been baked or cooked in food)
  • How much cannabis you have been using
  • Various factors relating to your unique body

Read on to find out more about how long marijuana can be detected in your urine, blood, saliva and hair.

 How Long is Cannabis Detectable via Drug Testing?

Cannabis can be detected in your system using a drug test. How long it can be detected after you last used the drug can depend on the method of sampling.

how long weed stays in your system table


Information published by Drug and Alcohol Information and Support Ireland suggests that, typically, cannabis can be detected in your blood for around 1-2 days after use. 

The level of use can increase this significantly. A study by Therapeutic Drug Monitoring suggests that in some chronic users, the drug can still be detected in your blood for up to 25 days after your last use.


The Mayo Clinic Proceedings’ Clinical Interpretation of Urine Drug Tests suggests that marijuana can be detected in your urine anywhere from a few days to several weeks after your last use. This depends on the individual’s level of usage. Generally, the guidelines suggest that:

  • Single use can be detected up to 3 days after your last use
  • Moderate use (4 times a week) can be detected 5-7 days after your last use
  • Chronic use (every day) can be detected 10-15 days after your last use
  • Chronic heavy use (multiple times every day) can be detected up to 30 days after your last use

MedlinePlus states that urine testing is the most common type of test used for detecting marijuana.


Saliva is often the method of choice for roadside drug testing of drivers. Saliva tests can pick up cannabis use in your system anywhere from 24-72 hours after your last use.


THC makes its way into hair follicles via your bloodstream, making hair testing another option for finding marijuana in your system. A hair test could detect the drug up to 90 days after your last use.

Hair tests tend to be less reliable than other forms of cannabis drug testing. If you’ve been in contact with second-hand marijuana smoke, or your hair comes into physical contact with someone who has touched cannabis, it could find its way onto your hair strands and result in a false positive.

What Are the Varying Factors That Affect How Long Weed Stays in Your System?

The question of how long marijuana stays in any one person’s system depends on a number of factors. Some of these are related to you as a person and others relate to the extent of your use. Many of these relate to your ability to metabolise the drug, rather than anything to do with the test itself.

Here are some of the factors that might affect how long weed stays in your system:

  • Body mass index (BMI): People with more body fat will retain THC for longer. Individuals with lower BMIs have less capacity to ‘store’ THC in their bodies
  • Sex: Females tend to have higher levels of body fat than males, making it more likely that cannabis metabolises slowly in their bodies and is therefore, more likely to be detectable for a longer period of time
  • Genetics: Some people naturally have higher levels of metabolism, reducing the time that weed is detected in your system. On the flip side, others have lower levels of metabolism, meaning that weed can be detectable for longer in their bodies
  • Frequency of usage: Infrequent users will typically be able to ‘clear’ the THC from their bodies faster than chronic users
  • Potency and method of intake: More potent weed has higher levels of THC, meaning it may stay in your system for longer. Weed that is ingested is also thought to stay in your system for longer than smoking

Metabolising Cannabis

There is nothing reliable that would lead to a faster metabolising of weed in your system. Some weed detox kits are available, which claim to help you dilute (and mask the dilution) of drugs in your system. However, these kits are known to be unreliable.

How Long Do the Effects Last?

According to TalktoFrank, the effects of cannabis can be felt as little as 1-2 minutes after smoking it. When cannabis is ingested, it can take as long as 45 minutes to feel the effects. Generally, the ‘high’ you get from weed will last between 40 minutes to an hour or more.

Cannabis Addiction Rehab at Priory

Cannabis is the fourth most abused drug in the UK, with many thousands of people requiring treatment and support after developing an addiction. Cannabis misuse is a serious problem that can seriously debilitate someone’s life, but today, many effective treatments exist that allow people to make a recovery.

At Priory, we can deliver world-class cannabis addiction treatment, including detoxification (if this is required), residential rehab and outpatient therapy, throughout our nationwide network of addiction hospitals, clinics and wellbeing centres.

If you think you might have a problem with drug addiction, you can get a free addiction assessment with Priory. Available over the phone, online or in person, free addiction assessments cover:

  • Your history with the substance you are struggling with
  • Any issues in relationships, or family history with drug addiction
  • Any other mental health conditions, such as depression or anxiety, that may be playing a role in your struggle with addiction
  • Deciding upon the best course of treatment for you as an individual

Book your free addiction assessment, or call us on 0330 056 6023 for more information about how we help those struggling with cannabis addiction every day.

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