At least 17 people are dead and 56 are in hospital after using what is thought to have been poisoned cocaine.
The cases are clustered around several towns in Buenos Aires province, Argentina’s most populous.
Samples of the drug are being analysed but judicial officials said the most popular theory was that the cocaine was deliberately tampered with as part of a settling of scores between traffickers.
The dead are from the towns of Hurlingham, San Martin, and Tres De Febrero, but authorities have warned the death toll could rise.
One of the areas affected is Puerta 8, a low income neighbourhood on the outskirts of Buenos Aires
The race is now on to find the rest of the deadly batch before it is consumed.
The local government of Tres De Febrero warned people to discard any drugs they had bought recently, especially if they have symptoms of confusion, convulsions, or loss of consciousness.
A dozen people have been arrested over the incident.
San Martin Attorney General Marcelo Lapargo described the situation as “absolutely exceptional”.
He told news channel Todo Noticias: “If the situation has escalated and the nature of trafficking has changed to the point where this becomes commonplace, I hope I never see it again.
“The urgency is to remove the poison from the market, to stop it from being sold.
“There must be a lot of people with a bag in their pocket and the number of people hospitalised shows that the most important thing is to stop this extremely high risk.”