Nineteen deaths from a new unregulated stimulant drug known as speckled cross have been reported to the authorities in Northern Ireland, a pathologist has told an inquest.

The fatalities happened between 18 June last year and 6 February this year and a coroner described the effect of the toxic substance – which causes agitation, convulsions, overheating and death – as catastrophic.

Detectives from the Police Service of Northern Ireland (PSNI) organised crime branch are investigating, the Belfast hearing was told. Stormont's health minister, Edwin Poots, expressed concern about the number of new substances appearing on the streets and warned that users were risking their lives.

Coroner Suzanne Anderson held an inquest into the death of a postman from using 4,4-Dimethylaminorex. Hundreds more tablets, white with cherry imprints, were found at his house in Co Down.

Anderson said: "Hopefully action can be taken to stem the supply of these dangerous drugs."

Poots added: "The only safe advice is not to use them."

Brian Mills, 41, from Shore Road in Kircubbin, Co Down, complained to his son James that he was too warm following an all-night drinking session at his home in August last year. Minutes later he stopped breathing, appearing to be having a fit, James Mills told the inquest. Paramedics were unable to save him.

The coroner added: "I think this case highlights very clearly the grave risks involved in taking illegal drugs and I think following all these deaths, this cluster of deaths, I certainly am pleased to know that the police are taking the matter seriously and are liaising with Forensic Science Northern Ireland (FSNI) and with the pathologist's department."

Dr Bernadette Prentice, a scientist from FSNI, analysed the victim's blood for toxins and found 4,4-Dimethylaminorex, known as speckled cherries or speckled cross.

A total of 287 tablets containing the drug were discovered at Mills's home by his brother, Leonard. They were confiscated by police.

Dr James Lyness, assistant state pathologist in Northern Ireland, said Mills's death was one of several involving the drug.

He told the inquest: "There are 19 cases from 18 June 2013 to 6 February 2014 where the 4,4-Dimethylaminorex … is mentioned within the cause of death."

Prentice said the drug was not banned in the UK.

She added: "4,4-Dimethylaminorex can be regarded as a novel psychoactive substance. It has rarely been encountered as a drug of abuse and consequently [there is] very little data."

She said it could be sold as powder or tablets and was first found in the Netherlands in 2012 and later in Finland, Hungary – where there have also been deaths reported – and Denmark.

Police made several seizures of the substance in Northern Ireland last year and the PSNI's organised crime anti-drugs unit is investigating, Prentice told the inquest. She said police had been holding meetings with forensic scientists and the pathologist's department.

The toxicologist said a certified reference standard for testing was not available but there was little reason to doubt the accuracy of the results.

"It is clear that they will have some similarities to related stimulant drugs."

The coroner observed: "It seems to have had catastrophic effects from the outcome of our hearings in these inquests."

Other stimulant drugs similar to ecstasy, taken in Mills's case, were likely to have further increased the toxicity of 4,4-Dimethylaminorex, the inquest was told.

Constable David Hayes attended the scene of Mills's death. He seized four white tablets and later received 287 pills from Leonard Mills, who found them in his brother's kitchen. He confirmed: "C1 Organised Crime Department are involved in this matter."

Searches were made following Mills's death but nothing was found.

Poots said he was greatly concerned about the number of new substances reaching the streets and the harm they could cause.

He said: "There were a number of potentially drug-related deaths in Northern Ireland over last summer. As soon as we became aware of them, and the substances involved, the chief medical officer – Dr Michael McBride – issued a number of alerts through his formal alert system and also through the Drug and Alcohol Monitoring and Information System that is used to inform all appropriate services and those who may come into contact with drug users."

Poots added: "Over the past couple of years there has been growing concern about what have been inaccurately labelled as legal highs. They are marked 'not for human consumption' for a reason as the sellers are trying to get around our existing drug laws and medicines regulations."