GPs are making too many mistakes when prescribing drugs to patients, the official regulator says.
A General Medical Council review said errors were being made for one in six people on prescription drugs.
Its study - based on 1,200 patients - found the elderly and the young were the worst affected.
But the report said many mistakes were only minor and some would have been corrected by the pharmacist before the patients were actually given the drugs.
Nonetheless, researchers said it was clear there was room for improvement and called for better training for GPs and more checks on their prescribing practices.
They also suggested the length of the GP consultation should be increased from 10 minutes to 15 to ease the time pressure on doctors.
Lead researcher Professor Tony Avery added: "It's important we do everything we can to avoid all errors."
Lack of monitoring
The most common type of error identified was incomplete information on the prescription, followed by problems with dose and timing of doses.
In total, 18% of patients experienced a mistake with at least one prescription over the course of the year.
But for the over-75s the figure increased to 38%, reflecting the fact they were often on a number of different medications at the same time.
Children under the age of 14 were also more likely to experience an error - something that was put down to the difficulty of getting doses right.
But the overwhelming majority of cases were not classed as serious, with only 4% of errors judged as severe.
These included cases where patients were given drugs which they were allergic to, and a lack of monitoring of potentially risky drugs such as warfarin, which thins the blood.
Health Secretary Andrew Lansley said the government was working with GPs to improve practices.
But he said patients should be reassured that even when GPs made mistakes, there were systems in place to make sure patients were not affected.
"The vast majority of prescriptions are checked by community pharmacists, who spot and put right any errors when they are dispensed."