Prescription cost savings worth £54 million could be achieved through greater efficiency by Northern Ireland GPs, an Audit Office report said.
Costs are well above other parts of the UK and comptroller and auditor general Kieran Donnelly proposed reducing the amount of fluctuation between the amount paid by different practices and cutting the average bill by a tenth.
With demand for prescriptions growing as the population ages, the auditor acknowledged that progress had been made in producing savings. The Department of Health has challenged some of the watchdog's calculations.
Mr Donnelly said: "The findings in this report demonstrate the extent to which GPs choosing to prescribe cheaper - but just as clinically effective generic medicines - can lead to real savings.
"This is all the more important with demand for prescriptions rising year on year as the population ages and more and better treatments become available.
"It is important, therefore, that the Department and the Health and Social Care Board continue to support cost-effective prescribing to get the most from the significant investment involved."
Last year, community pharmaceutical services cost £460 million and pharmacies dispensed almost 39 million prescription items
The cheapest GP practice (£26,303) was half that of the most expensive (£55,501).
Spending disparities may be due to differences in the amount of prescribing; in the choice of drugs prescribed and their cost; or a mixture of both, the report said.
It added: "The precise causes of the variations require careful inspection to determine the extent to which they represent good quality practice."
The report said if practices with above-average prescribing costs reached the average of £41,004, efficiencies of around £19 million could be achieved.
The Department has commented that such an estimate is crude and does not take into consideration other factors associated with prescribing, such as access to other services; the impact of cross-border workers and private healthcare.
The document said: "The Department considers that there will always be a degree of variability between GP practices and therefore the full quantum of such efficiencies will not be realisable.
"Despite this, we consider that a rolling target could be set to minimise the level of variation between GP practices. Further we consider that there is scope to reduce the average over time.
"For example, reducing the average by 10% over a three-year period would generate savings of £54 million."
The Department has said it it does not accept the analysis or conclusions surrounding that total without further robust analysis.
The audit also compared local prescribing costs with the rest of the UK. While acknowledging that variation in practice and the definition of data may affect the interpretation of such comparisons, it calculated that if the prescribing costs of local GPs had been in line with those in Wales in 2013 there was potential to save up to £73 million.
A health department spokeswoman said efforts were going on to ensure that proper procedures continue to be applied to the management of public funds and in delivering improved health and social care.
"The health minister's underlying objective is to protect and improve health and social care services, ensuring that they are safe, effective and focused on the patient and client.
"Whilst clearly the contexts will be different in two jurisdictions, the Department acknowledges that the report has identified that further efficiencies may be possible, and it welcomes the Audit Office's acknowledgement that, through the efforts of departmental and health and social care staff, the real-terms cost of prescribing has reduced by 18% in the period from 2006 to 2013."
"In the four-year period to 2013/14, the health service has delivered £132 million in prescribing efficiencies.
"The Department will continue to seek out further deliverable efficiencies through its ongoing engagement with the Health and Social Care Board, GPs, pharmacists and the pharmaceutical industry."
Derry GP Tom Black, chairman of the BMA's GP committee in Northern Ireland, said the number of prescriptions has increased almost 15% from 34 million to 39 million scripts since 2010.
He praised doctors' efforts to economise.
"This success in saving money is unequalled in the health service."
The report suggests further ways in which GPs can save money, including use of generic drugs and reducing the amount of specific expensive drugs.
Dr Black said: "We are at the limit of generic prescribing. There also needs to be a look at hospital prescribing and how savings could be made there.
"There is an issue with wasted medications and the fact that all drugs, even low-cost, over-the-counter medications, are available on prescription.
"General practitioners across Northern Ireland will continue to prescribe with the well-being of their patients at the heart of everything we do.
"If we are to be required to make further savings, then that will require close working with pharmacists. Family doctors are happy to do this, but this approach will need to be fully resourced in order to work."