Some patients are ordering more medication just in case it is needed later, rather than because it is necessary at the time, the NPA said.
It added that patients on complex medication regimes are more likely to return unused medicines or unopened boxes.
The Buckinghamshire, Oxfordshire and Berkshire West Integrated Care Board (BOB ICB) previously said it was spending about £10m a year on medicines that go unused.
The NPA believed more checks for patients on several repeat prescriptions would help to reduce wastage.
“If a patient has any concerns about their medicines, they should speak to their pharmacist who can help them get the most out of their medicines,” Picard said.
The BBC has contacted the NHS for a response.