European cities risk higher numbers of heroin overdoses as Afghanistan's record opium poppy crop floods cities with the drug, the UN has warned.
Poppies grow in most Afghan provinces
In a letter to European mayors, the head of the UN's Office on Drugs said it was likely more users would die.
Antonio Maria Costa said that an increase in supply tends to make the drug purer and hence more dangerous.
Mr Costa urged Europe's mayors to take precautionary measures ahead of the surge in the drug on their streets.
Europe has traditionally been the biggest market for Afghan opiates and opium cultivation in Afghanistan increased by 59% this year.
"Some cities take the problem more seriously than others. Illicit drugs are a serious threat to our young people and the very future of our societies," Mr Costa said in a statement.
Mr Costa said he has strongly encouraged the mayors and Europe's community drug centres to be on the alert and take every possible measure to deal with the threat.
Europe's politicians, he said, should take responsibility for what is happening in their own backyards, rather than expecting their drug problem to be solved by others.