New regulations aimed at stopping people driving while on drugs have come into force in England and Wales.
Drivers face prosecution if they exceed limits set for the presence of eight illegal drugs, including cannabis and cocaine, and eight prescription drugs.
Police will be able to use “drugalyser” devices at the roadside.
Campaigners welcomed the move and ministers said it would save lives, but a motoring lawyer said the impact on prescription drug users was a concern.
Meanwhile, Greater Manchester Police says it will delay implementing the new rules for about two weeks as it needs more time to train its officers and examine the legal implications of the changes.
The new rules run alongside the existing law, under which it is an offence to drive when impaired by any drug.
The existing penalties mean drug drivers already face a fine up to £5,000, up to six months in prison and a minimum one-year driving ban.
The new regulations set low levels for the eight illegal drugs, with higher levels set for eight prescription drugs, including morphine and methadone.
Those using prescription drugs within recommended amounts will not be penalised.
Police will be able to use “drugalysers” to screen for cannabis and cocaine at the roadside.
Officers will also be able to test for these and other drugs including ecstasy, LSD, ketamine and heroin at a police station, even if a driver passes the roadside check.
New devices that can test for a greater number of drugs at the roadside will be developed in the future.
David Taylor, professor of psychopharmacology at Kings College, London and a member of the Department of Transport’s advisory panel on drug driving, said the rules would work as a much stronger deterrent and make prosecutions much easier.
Speaking on Radio 4’s Today programme he said: “It’s a zero-tolerance approach.”
He said any exposure would render people over the limit and would leave them over the limit for up to 36 hours.
The situation for people who use prescription drugs had not changed, he said, but “the onus is on the individual to assure themselves that their driving ability is not impaired”.
He recommended that drivers who take prescription drugs carry proof so they could produce it at the roadside if needed.