The NHS may be missing out on millions of pounds of reimbursement for cancer drugs because of onerous paperwork, say researchers.
Schemes to share the cost of expensive new drugs with pharmaceutical companies are becoming increasingly common.
Yet a survey of 31 English health trusts showed up to 50% of the costs had not been recovered.
The Department of Health said it was working to make the schemes as simple as possible.
Writing in Clinical Pharmacist, Steve Williamson, consultant pharmacist at Northumbria Healthcare NHS Foundation Trust, said he welcomed wider access to cancer drugs.
But added that the complexity of reimbursement procedures risked making schemes unworkable.
With more of these deals being put in place, the NHS is coming under increasing pressure to track patients, fill in the correct forms and meet the deadlines set by pharmaceutical companies, he said.
The survey asked pharmacists for details of refunds on four of the first so-called 'patient access schemes'.
It included bortezomib for multiple myeloma, in which the NHS is entitled to a refund if patients do not respond to treatment, and sunitinib for kidney cancer, where the first cycle is free followed by a discount.
The responses showed that for these two drugs, refunds may not have been received in up to 50% of cases.
Seven-in-10 respondents said that they did not have capacity to take on any more schemes.
The researchers said missed payments were in the thousands, and could even add up to millions.
Mr Williamson said the NHS needed to set some basic templates for how such schemes should work rather than each company coming up with their own paperwork.
And he said there needed to be more flexibility, with time-limits on reimbursement.
"The bottom line is we're delighted to have these schemes because they allow patients access to drugs but they have not been executed very well.
"For example with bortezomib, in principle this is a very good scheme because if it doesn't work you get the money back, but if for some reason you miss making one claim within the timeframe allowed it costs £12,000."
Survey co-author David Thomson, lead pharmacist for Yorkshire Cancer Network, said: "I would continue to support any mechanism that improves patients' access to effective cancer drugs at cost-effective prices to the NHS.
But he said an "inappropriate amount of NHS staff time" is being diverted from clinical roles into dealing with bureaucracy.
He also raised concerns that proposals for a £200m cancer drug fund for treatments not available on the NHS would further increase bureaucracy.
A Department of Health spokesman said it continued to work with the pharmaceutical industry to make the schemes as easy as possible to implement.
"We have - through the National Institute for health and Clinical Excellence (NICE) - now set up an expert advisory panel with strong NHS representation which is consulted when schemes are proposed and which provides advice to the department on their implementability.
"Primary care trusts and hospital trusts need to agree locally between them how any rebates paid by drug companies are dealt with."