Understanding Fentanyl and Its Effects on the Body

Fentanyl is a very strong painkiller that is typically used for severe pain such as following a serious injury, following a major operation or the pain associated with cancer. It is only available on prescription in the UK and comes in a number of formats, including lozenges or tablets, patches, nasal sprays and injections that are only usually delivered in a hospital setting.¹

Fentanyl is controlled as a Class A drug when not used medically under a suitable prescription.² That hasn’t prevented it from being increasingly used as a recreational drug, however. In the US, the situation has been described as an ‘epidemic’, with close to 75,000 overdose deaths linked to the drug in a single year.³

Fentanyl abuse is not as widespread in the UK but it is still a problem and the strength of the drug means it is relatively easy to overdose on. It is also very addictive and can build up in the system. If you do use the drug, it’s important to understand how it works and how it is metabolised and expelled. There may also be situations where an individual is asked to undergo a test for fentanyl at work or for legal reasons.

So how long does fentanyl stay in your system?

How Does Fentanyl Work in the Body?

Fentanyl is a strong synthetic opioid. This means it works in a similar way to naturally occurring opioids like morphine, codeine and the illegal drug heroin. It is around 50 to 100 times more potent than morphine though – which makes it a very effective painkiller but very dangerous if misused.⁴

As with other opioids, it works by affecting opioid receptors in the brain. It essentially blocks pain signals between the brain and the rest of the body. This can also work to reduce the anxiety and stress caused by pain but there can also be side effects including euphoria and a sense of extreme relaxation. These effects are commonly sought by people who use fentanyl recreationally.

Fentanyl Half-Life: How Long Does Fentanyl Remain Active?

The half-life of a drug is the time it takes for the amount of that drug’s active substance in your system to reduce by half. Fentanyl has a half-life of around seven hours, although this can be affected by on factors including the individual’s metabolism, method of administration (such as injection, patch or lozenge), and dosage.⁵

This means that, while the noticeable effects of fentanyl may only last a few hours, half of the active elements will still be in your system after seven hours and a quarter will still be in your system after 14 hours. Small or trace elements of the drug may still be detectable for a long time after that.

Fentanyl Detection Windows for Different Drug Tests

There are a number of types of drug test that are commonly used, some of which can be used to check for fentanyl. These include:

Urine Test: This is the most common method of testing for fentanyl and many other substances. It will typically detect fentanyl in the system for up to 72 hours after usage, although fentanyl metabolites have been detected up to 4 days after use.⁶
Blood Test: Less commonly used but may be used in specific medical situations. It typically detects fentanyl for up to 48 hours after use.
Saliva Test: Fentanyl is not easily detectable in saliva, so this test is not generally considered suitable for this drug. Researchers are currently working on a saliva test.⁷
Hair Test: Fentanyl traces can be detected in hair follicles for up to 3 months, but this test is usually only used in specific forensic or legal situations.

Fentanyl and other synthetic opioids will not usually show up in general drug tests and need to be specifically targeted.

Factors That Affect How Long Fentanyl Stays in Your System

There are a number of factors that can affect how long fentanyl stays in the body and how long it remains detectable. These can include:

Dosage and frequency of use
Method of administration (orally, intravenously etc)
Individual metabolism, overall health and hydration
Age and sex
Body mass (weight) and fat per centage
Liver and kidney function

Why Detection Times Matter for Recovery and Drug Testing

Fentanyl’s detection times can be important for individuals in addiction recovery, especially for those in outpatient programmes that involve regular drug testing. Awareness of detection times can also support safe detox plans, as fentanyl withdrawal can be severe and is advisable to undertake under medical supervision.

Outside recovery, an understanding of how long fentanyl stays in your system can also help minimise risks of overdose. Due to its strength, overdose is serious risk when fentanyl is misused and two milligrams of the drug can be fatal depending on a person’s body size, tolerance and past usage.⁸Using fentanyl before a previous dose has been fully metabolised can increase the risk by adding to build-ups of the drug already in the system.

Detoxing from Fentanyl Safely

All opioids are potentially addictive and can result in severe withdrawal symptoms if a dependency develops. Fentanyl withdrawal symptoms can be particularly intense due to the potency of the drug. Clinical and survey data suggest that fentanyl withdrawal is more severe, longer lasting and has a faster onset than heroin withdrawal, for example.⁹

Withdrawal symptoms can include:

Nausea and vomiting
Anxiety
Insomnia
Hot and cold flushes
Perspiration
Muscle cramps
Watery discharge from eyes and nose
Diarrhoea

Monitoring and medical supervision can help people get through the difficult period of fentanyl detox safely and more comfortably. For moderate and severe withdrawal, medications such as clonidine, buprenorphine, methadone or codeine phosphate may be prescribed.¹⁰

Rehabilitation centres and detox clinics can provide a safe and suitable environment for this process, as well as holistic recovery programmes aimed at addressing the psychological aspects of addiction and the root causes of substance misuse.

If you are worried about fentanyl misuse or any other substance, get in touch today for confidential help and advice.