Synthetic opioids are a range of substances that are synthesised in laboratories. They act on the same receptors in the brain as traditional opioids, such as morphine and heroin, to produce analgesic effects. Heroin fortified with synthetic opioids, such as nitazenes, and adulterated with xylazine was an evolving trend in the UK’s opioid market in 2023. 

https://www.nationalcrimeagency.gov.uk/threats/nsa-drugs-2024?