A group of more than 25 senior doctors have written a letter voicing concerns about increasing health-related problems from cannabis use.

In the letter published in today's Irish Times, the Cannabis Risk Alliance argue that society has "taken its eye off the ball" in relation to the harmful effects of the drug.

Adolescent Addiction Psychiatrist Dr Bobby Smyth is one of those who signed the letter.

He said they signed the letter because, as doctors, they see the risks and harms of cannabis on a day-to-day basis.

Dr Smyth believes people have lost sight of the risks and harms, which the group thinks have been escalating in the last 15 years, but are largely being ignored.

From the time he started working with the Adolescent Addiction Services around 16 years ago, Dr Smyth said the type of cannabis being used has changed.

"Cannabis that was used at that time, going back into the 80s and 90s was what we called hash, which was relatively low strength and wasn't causing huge damage to the teenagers I was encountering at the time.

"But its changed massively in the last six to eight years, its been replaced almost completely by a different format of cannabis called weed. It comes from the same plant but its way, way stronger."

He said he is stunned by the devastation this type of cannabis is causing.

"I would never have guessed that cannabis could cause this amount of impairment and dysfunction in a teenagers life," he said.

Dr Smyth said its the only drug that teenagers report using from first thing in the morning until last thing at night.

"You know 15 years ago people were spending €20 and that would get them through the week, now teenagers are spending €50 a day on cannabis and you know 15 year olds don't have that kind of money.

"So they're rapidly getting into very significant debt, that brings with it some degree of threat and intimidation and stress. And they deal with stress and intimidation by using more cannabis."

The idea of quality control by legalising cannabis is just a fantasy, according to Dr Smyth.

He said there is no evidence to back up the idea that legalising cannabis will eliminate sales of potent versions on the black market.

He said the sad reality, in places like California and Canada where cannabis has been legalised, is that people are still going to the black market to buy it.

In relation to it being less harmful than alcohol, Dr Smyth said it is a selective focus on certain specific harms.

He warned that harms they discuss in the letter focus on the risk of development of psychotic illnesses in adolescents who use it, cognitive impairment and dependency for young people.

He acknowledged there is a positive side for the use of cannabis in terms of medical advances, adding there is merit in continuing the study of cannabinoids, which are found in part of the plant.

However, he said, his concern is that these elements used for medicinal purposes could be misinterpreted by people into thinking the entire plant is a medicine and it is not.

"It's causing the public to think that the plant is not just harmless, that its actually good for you, which is then causing more people to choose to use it, which is then causing a subset of those to become dependent on it. So there's definitely merit in continuing the study of the cannabinoids and their medical benefit.

"But the worry is that that evidence has been used to soften up the public's attitude towards cannabis in preparation for legalisation which is what happened in Canada and California".

Dr Smyth said as a group of doctors they want to focus on the harm of cannabis.

He said he sees the merit to decriminalise cannabis in the sense of continuing prohibition but moving away from a criminal sanction.

However, he is concerned people could use the decriminalisation debate to slide into the legalisation agenda which is very different.

Another signatory of the letter, Dr Ray Walley, said more education is needed in schools in relation to the dangers of cannabis.

A full-time GP based in north Dublin, Dr Walley said they are also calling for more research on the benefits on cannabis for medicinal use and an evidence and scientific-based debate on the issue.

Speaking on RTÉ's Today with Sean O'Rourke, he said the group will be writing to the Minister for Health, the Oireachtas Committee on Health and a number of colleges, including the Royal College of Physicians and the Irish College of General Practitioners.

People Before Profit TD Gino Kenny said he thinks it is extraordinary that the doctors who signed the letter use phrases like "one-sided debate", "dishonest", "hi-jacking the truth" and said its an insult for people campaigning access to medicinal cannabis.

He said campaigner Vera Twomey, mother of eight-year-old Ava Barry, who has been granted a licence to use medicinal cannabis for her epilepsy, is repulsed by some of the terminology in the letter.

Ava has a rare, drug-resistant form of epilepsy called Dravet Syndrome, which can result in up to 20 seizures a day.

Ms Twomey invited Dr Walley to visit her daughter, who she said is thriving and attending school.

She said its deeply distressing that they had to fight hard for their daughter's life and get access to medicinal cannabis, and it is insulting to see a group of doctors talking about "trojan horses".

Both Ms Twomey and Dr Wally agreed that consultants need to be better educated on the uses of medicinal cannabis.