Heroin addicts are being advised to smoke the drug rather than inject it, following warnings they could easily overdose on high-strength batches being sold in Oxford.

The drug caused 12 users to overdose at the weekend. Nine were admitted to hospital but later discharged.

Detectives say they do not know where the drug came from or how much is in the city and are speaking to users to build up intelligence about who is supplying it.

Peter Penny, services manager of drugs charity Cranstoun Libra, based in Cowley Road, said: "The best harm-reduction advice is don't use heroin -- but that's not going to be a choice for many who already use it.

"Someone is far less likely to overdose if they smoke it rather than inject. If someone is going to inject they should use half their usual dose.

He added: "If somebody is with a person that overdoses they should put them into the recovery position and call an ambulance."

Police would only accompany paramedics if there had previously been violence towards ambulance staff at that address or if someone had died, he said.

Police said drug users who overdose would not be arrested as their priority was catching the dealers.

Ben Stoker, of Kidlington-based drugs charity Energy & Vision, said he hoped the worst of the problem had passed.

Thames Valley Police drugs co-ordinator Pat Peters said: "We really need to know where this batch has come from. We need information about suppliers and dealers."

Anyone with information should call the police on 08458 505505 or Crimestoppers on 0800 555111.