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Whoonga is a blend of detergent powder, rat poison and crushed antiretroviral drugs that are distributed free to HIV patients.
The drug is commonly used in many townships, particularly in and around Durban, and has proved to be popular among young people.
Provincial police spokesperson Captain Thulani Zwane said a mob went to the house of the man suspected of selling whoonga to young people.
"When they arrived at this house they set it alight. They then went to two other houses where they took out mattresses and burnt them.
"It is reported that they then came across a man, whom they badly assaulted. The man is in critical condition in hospital," Zwane said.
He said a case of public violence was being investigated and no arrests had yet been made.
Zwane warned the community not to take the law into their own hands.
"If they know or have information on people selling whoonga or involved in any criminal activities, they must report them to the police because by taking the law into their own hands they are breaking the law and would be arrested," Zwane said.
A member of the community, who, however, refused to be named, said they were fed-up with people who were "destroying the lives of children" in the area.
"This drug is destroying our young people and we cannot sit and fold our hands when people are selling it like it's nobody's business.
"We know that by acting in the manner we did (taking law into own hands) we broke the law, but we could no longer take it," he said.