Heroin described as "extremely toxic" and "lethal" has been in circulation in a Pembrokeshire town.
There have been reports that the dangerous batch of drugs has been available in the Pembroke Dock area within the last week.
It has been warned that the potential use of the drug can "significantly increase the risk of experiencing an overdose", which as a result could be life-threatening.
Dyfed Drug and Alcohol Service (DDAS), which provides drug and alcohol treatment services, urged anyone who may be affected to reach out and speak confidentially.
Sian Roberts, manager at DDAS, said: "We have become aware, via our working partnership with Dyfed-Powys Police, that there are reports of extremely toxic and lethal heroin in circulation in the Pembroke Dock area.
"The potential use of this drug can significantly increase the risk of an overdose, which could be life-threatening. We urge anyone who may be affected by substance use, to reach out and speak confidentially to DDAS by calling 03303 639 997 or access Barod’s Live Webchat Service at https://barod.cymru/.
"In addition, if anyone uses opioids, like heroin, or knows someone that does, we strongly advise them to not use out of sight of others and have naloxone available, in case of an overdose.
"Naloxone can temporarily reverse the effects of an overdose, and can save someone’s life. For more information on how to effectively respond to an overdose, to get hold of naloxone and know how to administer it effectively, again, please get in touch with DDAS."
Dyfed-Powys Police confirmed that a man had been arrested in connection with the bad batch of heroin but he has since been released.
A spokesman for the force said: "Following information regarding a bad batch of heroin in the Pembroke Dock area last week, officers have arrested a male, who has since been released under investigation. Enquiries are ongoing.
"It is important to note that these drugs may still be in circulation. We would appeal to drug users to seek medical attention immediately should they become unwell.
"Please share this information with anyone that you believe could come into contact with these drugs. To seek advice and support, visit https://barod.cymru/where-to-get-help/west-wales-services/ddas-dyfed-drug-and-alcohol-service/.
"Please be aware that some services may operate an automated service outside office hours. In an emergency, or if you think someone’s life is at risk, always dial 999."
This comes after two people died and three needed intensive care treatment after a sudden rise in drug overdoses in Powys, thought to be linked to fake benzodiazepines sold through the 'dark web'.
