A self-styled doctor and three of his patients died in a south Indian village after drinking a poisonous herbal medicine which he claimed cured diabetes and hypertension, officials said Tuesday.

Another patient is in a critical condition at a government-run hospital after they drank the concoction prepared by the doctor at his makeshift clinic in Alagappuram village in Tamil Nadu on Sunday.

"They were unconscious and vomiting and were shifted to hospital where four of them died," said local police officer Muthiah, who only uses one name.

The doctor, named by police as Muthupandi, first drank the herbal mix to prove its efficacy before giving it to five of his patients from the village, the police officer told AFP.

Muthupandi claimed to run a clinic in the nearby town of Tenkasi and was visiting villages in the region to treat patients.

The officer said police suspect that Muthupandi was only posing as a doctor and had a criminal background. Detectives have already ordered a thorough investigation into his credentials.

"There are few theft cases against him but nothing is conclusive. We are checking if he had any medical qualification" he said.

Government doctor Jasline Rajiah said viscera samples had been sent off for analysis at a laboratory to establish the cause of dead.

"It apparently looks like a case of poisoning," Rajiah told AFP.

Thousands of Indians mostly in rural areas fall prey to people posing as doctors in a country where public healthcare system remains dismal.

Many claim to be traditional practitioners of Indian traditional medical systems like Ayurvedic and Siddha which rely heavily on herbal medicines.