Synthetic opioids

Also known as

MT-45, AH-7921, W-18, W18


Synthetic opioids are man-made drugs that mimic the effects of natural opioids (such as opium or heroin). Synthetic opioids, like all opioids, can reduce feelings of pain, can produce feelings of euphoria, relaxation and sleepiness and can be addictive.

They can also suppress and even stop breathing and have been linked to a number of deaths across Europe, including in the UK, and the rest of the world.

Some synthetic opioids have medical uses, fentanyl is sometimes used to treat pain associated with some cancer treatments, but others, like MT-45 and AH-7921, have no known medical or non-medical use.

Some synthetic opioids are more potent than natural opioids, such as morphine, and can be effective at very small doses, which can make it easier to take too much, to overdose and die.

Medical synthetic opiates, such as fentanyl, are available as patches, pills, tablets, lollipops and solutions for injection [1].

W-18 is a synthetic, opioid drug, also known as 4-chloro-N-[1-[2-(4-nitrophenyl)ethyl]-2piperidinylidene]benzenesulfonamide).

Synthetic opioid drugs are man-made copies of naturally occurring painkiller drugs, such as heroin, a drug which is made from certain strains of the poppy plant. Opioid drugs are most often used in medical settings as powerful painkillers, although the euphoria that is part of the effect makes them attractive to drug dealers and drug users as recreational drugs.

They are also typically highly addictive, producing tolerance quickly, and withdrawal if the person taking the drug for a period of time suddenly stops taking it. Opioid drugs, whether natural or synthetic, also carry a significant risk of overdose, and for that reason are highly risky to take without medical supervision. The amount of the drug that can cause death varies a great deal from person to person, and small changes, such as how long and how much of the drug has been taken, recent weight loss, and interactions with other drugs taken, can drastically change the risk of overdose in the same person [2].


Both AH-7921 and MT-45 have been sold as white powder, they have also been found mixed with synthetic cannabinoids in herbal smoking mixtures [1].

Mode of use

Synthetic opioids can be used in a similar way to natural opiates; they can be injected, sniffed, swallowed or heated and inhaled [1].


As with natural opiates the effects of synthetic opioids include -

  • pain relief,
  • euphoria or well-being,
  • relaxation,
  • sleepiness [1].

Users have also reported sweating, itching and nausea [1].


They are the same as the risks of natural opioids, however, as some synthetic opioids are more potent than natural opioids they can be effective at very small doses, which can make it easier to take too much and experience the negative and harmful effects, which can include -

  • constipation,
  • itching,
  • nausea and retching,
  • lethargy,
  • loss of consciousness and coma,
  • there is a greater risk of overdose and death if you mix synthetic opioids with other drugs that suppress breathing such as alcohol, benzodiazepines (like diazepam or valium), and/or other opiate drugs (like heroin) [1].
  • dizziness or fainting,
  • suppression of normal breathing, including respiratory arrest,
  • if you inject synthetic opioids you can do nasty damage to your veins and arteries, and this can lead to gangrene, blood clots/thromboses and to infections,
  • there are also risks from sharing needles, syringes and other equipment used for injecting - with the danger of developing serious tissue infections or spreading viral infections like hepatitis b, hepatitis c or HIV,
  • blurred vision and temporary hearing loss has been reported by some people who had taken mt-45, in one case the hearing loss reportedly lasted for more than two weeks [1].

There are also social harms that can develop with repeated use of synthetic opioids, especially if you become addicted. These social harms can include committing crime to afford to buy more synthetic opioids, disruption to your family life and other relationships, general poor health and social functioning including through loss of employment [1].

Sniffing or inhaling synthetic opioids may damage your throat and nose. Injecting can do nasty damage to your veins and arteries, which can lead to gangrene, blood clots/ thromboses and to infections.

There are also risks from sharing needles, syringes and other equipment used for injecting - with the danger of developing serious tissue infections or spreading viral infections like hepatitis B, hepatitis C or HIV [1].


It is possible that a synthetic opioid may be contaminated during its production and shipment or it may be mixed with another substance(s), such as sugar, starch or powdered milk, to increase its bulk and the seller's profits.

Testing has shown that MT-45 has been sold in combination with other drugs [1].


Can you get addicted

Yes. Animal testing and user reports suggest that, as with other opiates, you can build a tolerance and become dependent to AH-7921 and MT-45 and the same is likely to be true for other synthetic opioids [1].


Although some synthetic opioids have been legal in the past, many have been illegal for some time.

The synthetic opioids AH-7921 and MT-45 are already Class A drugs under the Misuse of Drugs Act due to the potential health and social harms associated with their use.

It's important to realise that since 26 May 2016, when the Psychoactive Substances Act came into effect, none of these drugs are legal to produce, supply or import (even for personal use, e.g. over the internet) for human consumption.

The synthetic opioids that were made illegal under the Misuse of Drugs Act, are still covered by that legislation. All other psychoactive substances not currently covered by the Misuse of Drugs Act now fall under the Psychoactive Substances Act [1].

What if you're caught?

  • If the Police catch you with synthetic opioids they are likely to take some action. This could include a formal caution, arrest and prosecution.
  • A conviction for a drug-related or other criminal offence could have a serious impact. It can stop you visiting certain countries - for example the United States - and limit the types of jobs you can apply for [1].

Did you know?

  • Like drinking and driving, it's illegal to drive if your driving has been impaired by taking drugs. With some drugs, you can even remain unfit to drive the next day. As well as this drug-impaired-driving offence, it's now illegal in England and Wales to drive over set levels for any of 17 named drugs (legal and illegal) in your body, whether or not you are impaired. Very low limits have been set for some common illegal drugs such as cannabis, cocaine and MDMA. You can get a heavy fine, be disqualified from driving or even go to prison.
  • Allowing other people to supply drugs in your house or any other premises is illegal. If the police catch people supplying illegal drugs in a club they can potentially prosecute the landlord, club owner or any person concerned in the management of the premises [1].

Mixing with other drugs

Mixing synthetic opioids with alcohol, or with other sedatives such as benzodiazepines, can have serious consequences: an overdose is more likely, and this can lead to a coma or respiratory failure and death [1].


In early 2016, W-18 hit the news in Calgary, Canada, when police seized the drug after it was implicated in a large number of drug-related deaths - over 200 people were thought to have lost their lives at least in part due to W-18. Yet the drug was developed many years earlier, in 1984, and was patented in Canada and the United States the same year, in anticipation of its potential use as a painkiller.

However, its legitimate use has never been established.

It was not until 2013 when W-18 first seemed to have been discovered as a designer drug, where it was marketed by drug dealers as a legal substitute for other recreational drugs. This approach of using drugs which have never been formally identified as illicit drugs is a way of working around the legal status of drugs.

These so-called designer drugs are substances which have effects that are similar to illicit drugs, but have not yet been identified as such, so drug dealers can get away with selling them, and even marketing them as "legal highs."

Yet it is considered to be highly risky to take, and, as with other drugs, it is just a matter of time before the authorities recognize the risks of such as drug, and take action to protect the public from such high risk drugs.

In 2014, W-18 was added to the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) list of New Psychoactive Substances.

In 2016, The government of Canada posted a notice online, informing interested parties of a proposal to schedule W-18 as well as its salts, derivatives, isomers, and analogues, and salts of derivatives, isomers and analogues under the Controlled Drugs and Substances Act, and its regulations [2].