Warning is issued on street Valium.
The illegal yellow pills are the same shape and size as the small, round blue Valium which have been reported in Caerphilly over the past few months, but they are missing a fake MSJ logo.
Drugaid Caerphilly says all illegal Valium may be sold as diazepam but it is known to contain various toxic and dangerous substances to bulk up content.
They are warning that illegal Valium use can lead to confusion, aggression and drowsiness and, when mixed with opiates or alcohol, can put users at risk of overdose, coma and even death.
Street Valium, nicknamed MSJs or benzos, is a Class Ccontrolled drug in the UK, meaning it is illegal to possess the tablets without a prescription.
Rowan Williams, service manager for Drugaid Caerphilly, said: "Misuse of illegal blue-coloured Valium tablets is already a recognised issue locally.
"These new illegal Valium tablets which are yellow in colour, are being sold as 5mg per tablet, whereas the blue MSJ is commonly sold as 10mg per tablet.
"However, dosage can differ greatly and the risk of overdose with these tablets, as with any illegal, non-prescribed drug, is always high."
For further information or advice, contact the Drugaid GOALS service 029 2086 8675 during office hours or DAN 24/7 on 0800 633 55 88.
* Synthetic opioids like methadone have been developed to wean people off an addiction to heroin