More than £500m is being wasted by the NHS on overpaying for a handful of drugs, an MP estimates.
A Liberal Democrat MP says the government is overpaying for drugs
Liberal Democrat John Hemming used government figures on three drugs to show the NHS was slow to react to falls in prices once drugs lost their patent.
The Birmingham Yardley MP said it showed the Department of Health was mismanaging money when many trusts were struggling to balance the books.
But the government said it did not recognise the figures quoted.
Mr Hemming's criticisms come as ministers are coming under increasing pressure over their management of NHS finances.
The health service in England is expected to record an overspend of at least £600m for last year.
Over 7,000 job losses have been announced in recent weeks as trusts try to get out of the red.
Mr Hemming used official figures on anti-cholesterol drug simvastatin, angina treatment amlodipine and lisinopril, which is used to treat heart disease.
He calculated that government overspend on simvastatin alone was by £382m from 2002 to 2005.
For lisinopril, it was £106m, while £26m too much was spent on amlodipine.
Patents on all three drugs expired during the period, but Mr Hemming said the subsequent drop in prices was not reflected in the amount the government pays pharmacists for them.
Mr Hemming said: "The government is slow to react to drops in drug prices once patents end.
"Therefore, the money they are paying to pharmacists is way too much. At time when there are real funding problems this is mismanagement of the worst kind."
He said he was writing to the National Audit Office to ask for an investigation into this.
"These are only three drugs, there are no reason we won't see the same trend with others."
But the government said it monitored prices to ensure it was not overpaying.
A spokeswoman said: "The Department of Health does not recognise the claim that there has been a loss of £500m to the NHS in respect of these medicines.
"We have monitored prices of generic medicines after patent expiry and taken action when necessary, to ensure that reimbursement prices reflect market prices."
And Sue Sharpe, chief executive of the Pharmaceutical Services Negotiating Committee, added: "We do not believe the report to be accurate.
"We have been working with the Department of Health to establish profit levels and ensure that pharmacies are neither over or underpaid for their NHS services."