Doctors have been urged to change how they prescribe medicines to stop £300m of drugs being thrown away each year.
Pharmacists say this would be easy to do - if doctors offered more tailored, personal advice to patients and stopped prescribing large amounts of drugs to cover long periods of time.
But doctors say patient demands make it hard to make such changes,
Behind the counter of Ash Soni's pharmacy in south London lie several yellow bags which illustrate the problem.
They contain drugs that have been handed back over a one-month period and are worth thousands of pounds.
"Currently we've got about six bags and two drums," Mr Soni says. "It all goes to the incinerator; we can't re-use any of these drugs because we simply don't know if they've been stored correctly."
He explains that there are many reasons patients may be stockpiling drugs.
Some people change their medicine on the advice of doctors, while others simply choose to stop taking the pills.
Official advice is to return unwanted drugs back to the chemists, but Mr Soni accepts that many people will hang on to them or simply throw them away.
"That stuff we have no idea about, and carries risk because it's disposed in landfill and can leach into our water supply."
Mr Soni is vice-chair of the English pharmacy board of the Royal Pharmaceutical Society and says patients should be more sensible in their attitude to what they need.
"Have a better discussion with your doctor about your medicines," he says. "We can do something about when patients simply haven't taken the drugs and they've continued to be prescribed, or they're just not appropriate".
He also believes doctors should stop prescribing large amounts of drugs to cover long periods, instead sticking to a regime of regular repeat prescriptions.
Following a government report into how much money the NHS wastes on medicines, published in December 2012, the NHS in England, Scotland and Wales has been working to raise awareness of the issue with a company called Medicines Waste UK.
The company's managing director Jan James said: "This is not about apportioning blame to any specific group, it's about working together so that we collectively reduce waste."
NHS England, which is implementing the government's action plan for medicines waste, has launched a project working across primary and secondary care in the south of England, which is estimated to have saved nearly £3m in its first year.
NHS England's deputy chief pharmaceutical officer, Clare Howard, said there were "many complex factors" behind the issue.
"These include the ways in which medicines are developed, doctors' prescribing habits, the ways in which "repeat" medicines are ordered, and of course patients' understanding of the importance of taking medicines as prescribed and ensuring they only order what they need," she said.
A campaign has been developed to remind patients of three key messages: only order the medicines you need; take your medicines with you into hospital and dispose of unwanted medicines safely.
The Royal College of GPs said: "Many patients expect a prescription at the end of their consultation, particularly antibiotics for common colds and infections that will get better naturally or respond better to other treatments, and this can make it difficult for GPs to prescribe appropriately.
"Prescribing has been designated a clinical priority and we have already produced a wide range of resources, with the Health Protection Agency to support GPs in this area."