More than 100 breast cancer patients in Cornwall and Devon could soon be prescribed the drug Herceptin.
Herceptin is already used for patients with advanced cancer
The decision by South West Peninsula Health Authority (SWPHA) follows the court victory of Somerset woman Barbara Clark to be treated with the drug.
Cancer charity CancerBACUP said it was a "breakthrough moment" and that other health authorities should follow suit.
Herceptin costs up to £30,000 a year for each patient and health managers said it could pose financial problems.
Dr Jim O'Brien, director of public health at SWPHA, said: "The cost of providing this will be approximately £4m which will be divided between the 11 primary care trusts in Devon and Cornwall and included in 2006/07 drug budgets."
Herceptin is already widely used on patients whose breast cancer is at an advanced stage, or those who are terminally ill.
'Hope and pray'
But on 5 October, following former nurse Ms Clark's court victory, it was announced all women with early-stage breast cancer in England would be tested to see if they were eligible for the drug.
Tests had shown it could greatly extend sufferers' life expectancy, but it had not been approved for use on women in the early stages of the illness.
The SWPHA decided to allow treatment on the condition that the treatment is supported by the patient's clinician, and that the patient herself is willing to receive the drug.
Dr Duncan Wheatley, a consultant who specialises in breast cancer for the Royal Cornwall Hospital, said there were risks, but compared with chemotherapy drugs it was "very safe".
One study had shown that Herceptin could halve the chance of a relapse for women after chemotherapy.
He said: "We expect results to get better and better and a lot more people will not needlessly die."
The drug's manufacturer, Roche, has said it is likely to apply for a licence for the use of Herceptin on early breast cancer next year.
Joanne Rule, chief executive of CancerBACUP, said: "Women all over the country will hope and pray that their primary care trust's (PCT) will follow suit.
"The secretary of state must now make it clear to all PCTs that they must fund doctors' decisions to prescribe Herceptin to any woman who could benefit from it."