In 180,000 cases, the error is so serious that it could kill the patient, a survey for the Royal Pharmaceutical Society has discovered.
Not surprisingly, the over-75s are at greatest risk because they are handed the most prescriptions. Four out of 10 elderly patients were handed a prescription during the year covered by the survey that contained an error.
The most common mistakes were prescribing an incorrect dosage - which, in itself, could be fatal - or failing to indicate any dosage.
The survey was based on 15 general practices around the UK, and included the analysis of 1,777 patient records and 6,048 prescriptions.
(Source: Royal Pharmaceutical Society).