Women who take Feminax for period pain or anyone taking Solpadeine will fail the new roadside test for drug driving, according to GPs.
Doctors say clarity is needed over whether gardaí can correctly assess if the person's ability to drive is impaired.
The new legislation means that if you fail the roadside saliva test and then fail the impairment test, you could be deemed unfit to drive and possibly face prosecution.
Dr Mark Murphy, chair of communications for the Irish College of General Practitioners (ICGP), said: "Care must be taken to ensure people are not incorrectly deemed to be unfit to drive, particularly older members of society."
The ICGP fully supports the initiative by the Road Safety Authority and the Medical Bureau of Road Safety to reduce harm on the roads and also to promote road safety.
"Our concern is over opiates and benzodiazepines which are two groups of drugs that will result in a positive test result," he said.
Codeine, which is part of the opiate family and is a popular pain killer, is found in Solpadeine, Nurofen Plus and Feminax.
"About 4pc of the population are on long-term use of opiates such as Solpadeine for chronic pain and they can tolerate it and do not feel it impairs them," Dr Murphy said.
"The key issue here is older drivers who are on long-term opiates for cancer pain and are able to drive. It is not a crime to test positive for opiates or benzodiazepines but it is a crime to be impaired while driving.
"As a doctor it can sometimes be very hard to assess if somebody is impaired and there is no hard and fast objected criteria in this.
"The anxiety for GPs is how can a garda correctly assess if somebody is impaired."
The Garda roadside test for impairment includes testing for pupil dilation, asking motorists to touch their nose with their finger, assessing balance, a walking test, and standing on one foot for a count of eight.
The other group of drugs causing concern for GPs is benzodiazepines, he said.
"GPs frequently prescribe them for two to three days if someone is acutely bereaved or has severe back spasm and we would counsel them not to drive for those three days.
"However, there are people who are on them for 30 years and take them at night-time. Whether this impairs their driving the next day is a grey area."
By and large he said the majority of people should be reassured and the new legislation "is a very positive initiative".
"We would also say it is essential that if someone is taking their prescribed medicine and they feel impaired or intoxicated, they should not drive.
"What is important is to clarify for gardaí - and the public - how exactly this test of impairment will operate. There maybe a role for some discretion by the gardaí and indeed they may value guidance on it," he added.
The other group of drugs tested for are heroin and cannabis; any detection of them will result in being immediately arrested.