The medical journal The Lancet has said claims for the anti-cancer drug Herceptin should be treated with greater caution.
Herceptin is not licensed for early breast cancer
In an editorial, the journal argued the evidence on effectiveness and safety is not yet reliable.
It warned political and media pressure should not undermine the usual process of checking new drugs.
On Tuesday Health Secretary Patricia Hewitt ordered an urgent review of a decision to deny the drug to a patient.
North Stoke Primary Care Trust (PCT) told Elaine Barber it was not convinced the drug was safe or cost-effective.
The Conservative Party has also called for national funding for the drug until it has been fully reviewed by the NHS advisory body, the National Institute for Health and Clinical Excellence (NICE).
Herceptin, which can cost £20,000 a year for a course of treatment, is currently licensed only for use in women with advanced breast cancer.
However, recent clinical trials suggest the drug might also be effective at treating women in the earlier stages of the disease.
NICE, which is to introduce a new fast track procedure for asssessing potentially life-saving drugs, is primed to assess Herceptin for these patients once a licence has been granted - but not before.
However, campaigners say the drug should be made widely available to women who stand to benefit now.
The Lancet said that studies into the effect of the drug on early stage breast cancer had only produced preliminary results, from which it was difficult to draw meaningful conclusions.
It targets the HER2 protein, which can fuel the growth of breast tumours
Herceptin prevents this process happening
Around a fifth of breast cancers are HER2 positive
It is currently licensed for use in women with advanced breast cancer - where the disease has spread within the breast or to another organ
Early stage breast cancer refers to the first occurrence of the disease
The cost for one year's treatment with Herceptin is £19,500
In its editorial, it said: "The best that can be said about Herceptin's efficacy and safety for the treatment of early breast cancer is that the available evidence is insufficient to make reliable judgments.
"It is profoundly misleading to suggest, even rhetorically, that the published data may be indicative of a cure for breast cancer.
"Drug regulatory agencies and bodies such as NICE play an important part in translating research evidence into clinical guidance.
"It is vital that their decisions are made carefully after considering the totality of available evidence.
"They must be free from political, special interest, or media influence, no matter how well meaning."
Assessment too slow
Pamela Goldberg, of the Breast Cancer Campaign, agreed that the debate surrounding Herceptin had been influenced by the media, and other opinion formers.
However, she said the debate had highlighted the fact that new drugs were currently assessed too slowly.
"It will be some years before we know the long-term side effects and benefits of Herceptin, but for some patients the short or medium term will be what counts - there will be no long term benefit for those women who do not survive."
Emma Taggart, of the charity Breakthrough Breast Cancer, said: "Many cancer experts, who have looked at the recent trial data, want to prescribe Herceptin for their patients and these decisions should continue to be made on a case by case basis and based on clinical judgement.
"Like any drug, Herceptin does have side effects and patients must be fully informed of these so they can, in partnership with their doctor, make informed decisions about their treatment."
Roche, which makes Herceptin, disputed The Lancet's claim that there were holes in the recent research into the drug's impact on early stage breast cancer.