The FDA will this month ban the oral intake of the drug for people and only allow it to be given to animals if prescribed by a veterinarian.

Colistin has been described as an “antibiotic of last resort”. It used to be sold over-the-counter at pharmacies, but public sale of the drug was curbed because of the risk of people getting infected with serious drug-resistant bacteria.

Reports that pigs were getting food laced with colistin has worried the public and doctors fear such “frivolous” use could lead to the spread of deadly bacteria resistant to antibiotics. Bacteria that can resist “last resort” drugs such as colistin are unlikely to be cured by other medicine used in early phases of treatment, officials have said.

FDA deputy chief Prapon Angtrakul said his agency and the Livestock Development Department had agreed to make colistin a controlled medicine for use only under supervision of a veterinarian – as per the fourth article of a strategy approved by the Cabinet to counter drug resistance and control the use of antibiotics in animals.

Use of colistin for humans would also be restricted to injections to prevent frivolous use, he said. Both changes would be tabled at the FDA board’s meeting this month.

“The residue of colistin in pork meat is [only] a small amount, so it would not directly harm consumers. But we’re worried that colistin in pig feed could lead to drug-resistant bacteria,” Prapon said. He urged people to properly cook pork meat so bacteria are killed and drug-resistant bacteria is eliminated.

Thai Drug Watch group manager and pharmacist Dr Niyada Kiatying-angsulee said uncontrolled use of colistin could lead to bacteria with a drug-resistant gene that could lead to many human deaths in the future.

Niyada cited a report that 96 used bottles of antibiotics were found at a farm with 300 pigs at Nakhon Pathom. She said the bottles had contained nine strong antibiotics including seven bottles of ceftriaxone, a bacterial antibiotic only given via injection to hospitalised patients, and unregistered colistin.

She said a study of a pig farm in China in 2015 had shown horizontal gene transfer of bacteria from animals to humans and vice versa. This led to the important discovery of a gene with mobilised colistin resistance (MCR-1).

Niyada said media in the United States reported the first MCR-1 gene in E coli bacteria last May, which was found in a Pennsylvania woman. The new caused global alarm about colistin “abuse”. Dr Pisonthi Chongtrakul from Chulalongkorn University’s Faculty of Medicine said the American case was alarming because E coli would be cured immediately by colistin so the case was unprecedented.

Pisonthi said feeding colistin to pigs without supervision by a veterinarian could lead to two dangers. First, if farmers slaughtered pigs for sale in less than three days when the body would naturally get rid of the drug residue, consumers could end up accumulating it and their kidneys would be affected.

Secondly, colistin was used to treat drug-resistant germs like Acinetobacter baumannii bacteria found on medical equipment so use of the antibiotic could cause a serious drug-resistant gene to be passed from incoming bacteria to other bacteria in people’s bodies, he said.

Pisonthi urged the authorities to stop people from being able to easily buy antibiotics over the counter, adding that many people used antibiotics wrongly and unnecessarily.

Meanwhile, Dr Athipoo Nuntaprasert from Chulalongkorn University’s Faculty of Veterinary Science said colistin should be put in water for pigs to drink when treating them, and such animals should only be slaughtered for sale at least 30 days later to ensure consumers’ safety. Athipoo admitting there was the possibility of bacteria developing resistance to colistin and long-used antibiotics, but said there had been no report of a drug-resistant bacteria transmitted from one pig to another yet. He said colistin should be given properly and only as a last resort.

Livestock Department chief Apai Suttisunk said the agency had controlled and monitored use of the drug while encouraging farms to operate according to international standards. Over 10,000 livestock farms have been certified for producing meat that was free of antibiotics or contamination, along with 2,700 shops that sell meat.

He said the department would punish farms that use illegal drugs or over-used medicine, adding it would also seek to control importers of livestock drugs and force manufacturers to abide by the law.

Article 12 of the 1967 Drugs Act listed penalties for people who illegally sell medicines – up to five years in jail and a fine up to Bt10,000. Article 72(4) listed punishment for those who sell an illegal drug – up to three years in prison and/or a fine up to Bt5,000.

The success of agencies in boosting bio-safety at farms had led to a gradual decrease in antibiotic abuse, Apai said. He added that the department would join relevant agencies to implement guidelines to control the use of veterinary medicines in livestock production, as well as abide by the national strategy to manage antibiotic-resistance from 2017 to 2021, which includes appropriate control of the use of antibiotics administered to animals.