Around one in 20 prescriptions written by family doctors contains an error, a medical watchdog has warned.
Around one in every 550 items was judged to contain a serious error, the research commissioned by the General Medical Council(GMC) found.
The most common of the prescribing or monitoring errors were lack of information on dosage, prescribing an incorrect dosage, and failing to ensure that patients were properly checked with blood tests.
One in eight of all patients had a prescription item with an error - this rose to four in 10 patients aged 75 years and older.
A number of factors were found to be associated with increased risk of prescribing or monitoring errors and these included the number of medicines a patient was taking (there was a 16% increased risk of error for each additional medicine) and the age of the patient (children and those aged 75 years and older were almost twice as likely to have an error as those aged 15-64).
Researchers concluded that causes included deficiencies in the training of GPs regarding safe prescribing, time pressure, and lack of robust systems for ensuring that patients receive necessary blood tests.
Despite these concerns, they found that GPs took prescribing very seriously and used a range of strategies to try to avoid serious errors.
Professor Sir Peter Rubin, chairman of the GMC, said: "GPs are typically very busy, so we have to ensure they can give prescribing the priority it needs.
"Using effective computer systems to ensure potential errors are flagged and patients are monitored correctly is a very important way to minimise errors."
Professor Tony Avery of the University of Nottingham 's medical school, who led the research, said: "Few prescriptions were associated with significant risks to patients but it's important that we do everything we can to avoid all errors."
The study took place in 15 general practices from three areas of England, regarded as reasonably representative of other general practices in England.
A total of 1,777 patients were included in the study. Their medical records were investigated to identify potential prescribing or monitoring errors associated with 6,048 prescription items issued in the previous 12 months.
Dr Clare Gerada, Royal College of General Practitioners chair of council, said: "There are over one million patient consultations in general practice every day across the UK, and this report demonstrates that in 95% of cases GPs prescribe safely and effectively in the best interests of their patients."