Pharmac is facing calls to re-think its rule giving patients three months' worth of certain medicines in one go.

Pharmacists have said they cannot keep track of what people are taking and pills are stockpiled at home or chucked in the bin - which builds a mountain of medical waste.

The medical sector estimates $40 million worth of pharmaceuticals are wasted every year, and Auckland pharmacist Warren Flaunty is no fan of the policy to prescribe 90 days' worth of pills.

"If you get three months supply of a prescription, the pharmacist has got no idea what medications you are taking, whether you are taking them correctly.

"You can't monitor somebody's medication. Pharmacists like patients to return monthly so that we know that they're picking up their prescription, they're taking their medication correctly, and we dispense the next month for them.

"A couple come in, the wife picks up 720 paracetamol, the husband picks up 720, so immediately they're home with 1500 paracetamol. But the government doesn't care because the price of the drugs are so low that the dispensing fee that we used to get is more than covered by the dispensing three month 'stat'," said Mr Flaunty.

Another experienced pharmacist, Bulls-based Graham Platt said no-one was keeping track of the unused drugs.

"Unused medicines in general I believe is a very, very big problem because the wastage is enormous because there doesn't appear to be any real policy on minimising wastage. And therefore these are left to be disposed of by the people who get them, or in some cases taken back to the pharmacy to be disposed of."

Pharmacists said ibuprofen, paracetamol and beta blockers were among the medicines covered by the three-month rule.

The drug-funding agency Pharmac defended the policy and said it made sense for the patient.

"Pharmac's funding rules allow most medicines to be dispensed in line with the amounts defined by Medsafe.

"This is because in most cases it's more convenient for people to receive their medicine all at once, rather than having to repeatedly return to their pharmacy to collect smaller batches," it said in a statement.

That view is not shared by Associate Professor Dr Barrie Peake, an environmental chemist at the University of Otago.

"One of the problems is this 'stat' dispensing, where pharmacies now dish out prescriptions for three months instead of say one month. And it means if someone gets over their illness for which they've been prescribed their medication, say after a couple of weeks, then they've got two and a half months of unused medication which has been prescribed to them.

"This is one of the main reasons why there is increases in unused medication not necessarily being retuned to pharmacies but certainly present in homes," said Dr Peake.

Lucy Shieffelbien of the National Poisons Centre said that medical wastage needed to be investigated.

"It's not just the environmental impact, returning drugs to a pharmacy. It also gives the pharmacy the ability to see what medicines are being used or aren't being used.

"So that can help inform policy, things like Pharmac purchasing of drugs, are we oversubscribing, are there patterns here, do people really need three months' supply at a time," said Ms Shieffelbien.

With advice from its clinical committees, Pharmac has promised to keep looking at what medicines should and should not be given out for three months' at a time.