Police have issued a warning amid worries a rogue batch of heroin may contain deadly fentanyl.
Synthetic opioid fentanyl is 100 times more potent than morphine and just 0.002 grams mixed with heroin can kill.
South Wales Police said the discovery was made after a drug addict tested positive for the substance following a non-fatal overdose.
Det Insp Stuart Johnson said: "I urge all drug users in the local area to be cautious."
A force spokesman said the batch of heroin was found in Neath Port Talbot, which has the highest rates of drug deaths in Wales and the second highest in the UK.