Pharmacists are pushing for a national disposal system for unused drugs, as an unknown quantity are ending up in the bin and down the toilet.
A recent white paper shows the cost of wasted medicines is about $40 million a year - about 5 percent of the country's $800m annual spend on pharmaceuticals.
But Pharmacy Guild board member Anthony Roberts said the true cost of wasted drugs was much higher because the estimate did not include the negative effects on health.
Mr Roberts said many people didn't take the medication they were prescribed, which was a drag on the health system because the resource wasn't being used to its greatest effect.
There was also a risk of people being poisoned.
"You may understand that you shouldn't be taking those [pills] ... but your grandkids don't."
Mr Roberts said the gold standard was for unused medicines to be incinerated but most were ending up in landfill and waterways, impacting the environment.
Dr Rhiannon Braund, clinical pharmacy lecturer at Otago University's School of Pharmacy, said the lack of a national disposal system was confusing for patients.
"Because we've got this haphazard approach throughout the country, we've got patients who're not bringing their medicines back," she said.
"I've heard of pharmacies who don't want their patients bringing the medicines back because then the pharmacy has to pay for the disposal.
Dr Braund said some DHBs were paying for proper disposal but there was no unified approach and other steps were needed to reduce waste, including more accurate prescriptions.