GP prescriptions rose by a third between 2004/5 and 2011/12 while costs fell 11%, said Audit Scotland.
Its report, Prescribing in general practice in Scotland, said the savings were due to improved prescribing support for GPs and lower prices for some drugs.
Audit Scotland said the NHS could save a further £28m by dealing with drug wastage and promoting generic prescribing.
GPC Scotland welcomed the findings and backed the report's advice that practices should be given further prescribing support from pharmacists.
GPs in Scotland generate 91m prescriptions a year, costing the NHS nearly £1bn. Costs rose by 50% in real terms in the seven years to 2004/5.
However, the Scottish government and the NHS had made efforts to improve the quality and cost effectiveness of prescribing, the audit report noted.
GP prescriptions rose by a third over the next seven years to 2011/12, owing to the ageing population, introduction of the QOF, a rise in clinical guidelines and new initiatives such as health checks. The report found that more than 900,000 patients aged over 50 are taking four or more different drugs. Yet spending on GP-prescribed drugs fell by £120m, or 11%, during this time.
Cheaper statins and better information and feedback on prescribing habits are thought to be behind the fall. Audit Scotland expects primary care to save a further £86m from expired drug patents in 2012 and 2013.
Caroline Gardner, auditor general for Scotland, said the NHS had 'significantly improved' how it manages spending on prescription drugs.
'Demand for prescriptions is likely to increase further and it's important that the NHS continues to work with GPs to make sure patients get the drugs they need and spending is well managed,' she said.
GPC Scotland chairman Dr Alan McDevitt said: 'Prescribing costs are second only to workforce costs in the health budget and savings here can have a significant impact on health spending.'
He said the NHS should not look simply to cut numbers of prescriptions, but should consider the wider impact of GP prescribing. He backed calls for more pharmacy advisers to support GPs on prescribing.