A health and social care regulator is calling for improved communication between hospitals and doctors' surgeries in order to prevent patients accidentally mixing medicines.
The Care Quality Commission (CQC) revealed that patients could be at risk from potential reactions to their drugs when leaving hospital.
It found that poor communication between NHS services could lead to patients being given drugs they were allergic to or which react badly with other medication.
Overall, 12 PCTs in England took part and 280 GP surgeries were surveyed.
The study also showed that 98% of GP surgeries gave hospitals details about medicine in non-emergency cases. But when broken down, only 24% were providing details about previous drug reactions, 14% about other illnesses a patient suffered, and 11% about known allergies.
When the CQC asked GP practices about the quality of information given by hospitals when they discharge patients, 81% said details of medicines was incomplete or inaccurate "all of the time" or "most of the time".
Almost half (47%) of doctors also complained that it took too long for hospital discharge summaries to arrive, meaning patients were seen without a full set of records.
Prescribing errors and a failure to review medication and near-misses were the fourth most commonly reported issue to the National Patient Safety Agency during 2008.
The CQC's chief executive, Cynthia Bower, said: "It is important that basic systems to share essential patient details are working effectively to get the right information to clinicians at the right time to minimise these risks."