Melanoma drug Keytruda's maker is accusing New Zealand drug-buying agency Pharmac of under-playing evidence about the drug's effectiveness.
But Health Minister Jonathan Coleman says the evidence the company is referring to has not been peer-reviewed.
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Pressure has been mounting on Pharmac to fund one of two new immunotherapy drugs for the treatment of advanced melanoma.
One of those drugs is Keytruda, made by Merck Sharp & Dohme.
Correspondence released to RNZ News under the Official Information Act shows Merck Sharp & Dohme has complained to Dr Coleman about Pharmac.
Read the OIA provided to RNZ News (PDF, 856KB)
The company said the latest clinical data it gave the agency prior to key committee meetings was not reviewed or reported in the agency's minutes.
"In the recent debate over the efficacy of Keytruda, there has been some confusion because the most up-to-date and correct clinical data has not always been referenced in the figures given to the media and the wider public."
The data it provided showed Keytruda offered a much greater benefit to patients than the clinical paper that the agency had reviewed, it said.
"The use of incorrect data gives a misleading and false impression of how effective this breakthrough drug is" - Excerpt from Merck Sharp & Dohme's letter to Dr Coleman,
But Dr Coleman said the evidence referred to by the company had not been peer-reviewed.
In a letter responding to Merck Sharp & Dohme's complaints, Dr Coleman said Pharmac did take "all available evidence into account when making its funding assessments" and that the people in the key committee meeting had considered all information from the company.
"I am advised that the information you refer to is from presentations delivered at clinical conferences and has not yet been subject to peer review," Dr Coleman said in the letter.
"I am also informed that some of the promotional material based on the information you refer to has subsequently been withdrawn and corrected internationally by Merck Sharpe & Dohme," he said.
Dr Coleman's office said it was not aware of any reply from Merck Sharpe & Dohme to that letter.
New drugs are effective - Cancer Society
The Cancer Society said there was already enough information available to justify funding the new class of drugs.
The society's medical director, Chris Jackson, told Checkpoint cancer experts believed Keytruda and the equivalent drug, Opdivo, were effective.
"When you've got a newer drug, you can always say 'well we'll always have more information in six months, we'll have more information in two years...' Of course you will.
"But is there enough information today, to recommend these new drugs as the new standard of care for patients in our care? And the answer, very clearly in the opinion of every oncologist I've spoken to, is clearly yes."
"They are clearly an improvement over what we've had before" Cancer Society medical director Chris Jackson