Motoring groups said the findings showed that a planned drug-driving law could not come soon enough.
A UK-wide poll of 2,000 people by price comparison website Confused.com found 19 per cent admitted to drug driving – 7 per cent using illegal drugs, and 12 prescription medication.
However, figures obtained from 30 of 46 police forces across Britain showed the total number of convictions in England and Wales, and offences reported by police in Scotland, fell by 12 per cent, from 1,294 in 2011, to 1,132 last year. Drugs involved included cannabis, heroin, cocaine and ecstasy.
Police Scotland said the number of people reported to prosecutors for offences, which included drug-driving, was 590 in 2011 and 523 in 2012.
The opinion poll also showed 27 per cent of those questioned knew someone involved in drug-driving. More than half of those who had been convicted had been caught twice, while one in five had offended three or more times. Young drivers, aged 18 to 24, were the most common drug-drivers, with one in four admitting to it.
Confused.com said prescription and over-the-counter drugs such as anti-depressants, painkillers, antihistamines and cough mixtures could have a sedative effect, but most users probably thought they were safe to drive. The survey showed around 6 per cent of those questioned still drove even when feeling ill after taking over-the-counter medicines.
The Institute of Advanced Motorists (IAM) said new testing equipment would help tackle the problem. Neil Greig, its Scotland-based policy and research director, said: “What is clear from the survey is the new drug driving laws currently being consulted on for UK-wide implementation are long overdue.”
There is a long-standing offence of driving while impaired due to drugs, which is difficult to prosecute because it is necessary to prove impairment. The proposed new charge will simply require a specified substance to be found in the body.
The Scottish Government said the new law would help convict drug-drivers. A spokeswoman said: “The ability to set drug driving limits will be a useful tool in helping to make Scotland’s roads safer.”