Doctors in mid and west Wales have been cleared to provide the breast cancer drug Herceptin on the NHS to women in early stages of the disease.
Protesters had complained at a post-code lottery for the drug
Swansea Local Health Board (LHB) made the announcement as a small group of protesters gathered on Wednesday.
Despite the decision, clinicians still have to decide whether their patients meet the criteria for it.
The LHB said it was following the advice of local experts while protesters welcomed the move.
It said it brought the region in line with all the other LHBs in Wales - the changes in mid and west Wales will come into force on 1 March.
Recent research in the US has suggested the risk of tumours recurring with certain type of breast cancer can be cut by up to 50% if Herceptin is taken in the early stages of the disease.
Although Herceptin has not been licensed for use in the early stages of breast cancer, the decision as to whether it is appropriate to prescribe will be left up to individual clinicians within agreed limits.
Wednesday's protest in Swansea had been organised by mother-of-two Julie Davies.
She was diagnosed with breast cancer last October and wanted to persuade the LHB to change its policy.
Speaking after the announcement she said: "We've got what we came here for.
"I was diagnosed in October and had an operation in November. Chemotherapy is on-going so its still a few months before a need it but I was not prepared to sit and wait until then.
"There were a number of ladies here this morning, all with the same condition, some who need Herceptin now.
"I've got time to play with but I'm sure it is a great relief to them."
Swansea LHB said all seven LHBs covering mid and west Wales had agreed to adopt the interim advice of the South West Wales Cancer network.
In a statement it said: "While Herceptin is licensed for use in advance cancer treatment it remains not only unlicensed for early use, but to date, is also not supported by national guidance.
"Nevertheless, as an interim measure, clinicians in mid and west Wales will now be able to act in line with colleagues throughout Wales and prescribe the drug subject to agreed parameters which cover the whole of the county.
"The agreement means that, where the patients in Swansea meet the parameters and where their senior clinicians believe the drug should be prescribed, it will be made available on the NHS from 1 March.
"There will be some cases, however, where the individual prognosis for patients do not meet the agreed parameters."
A Welsh Assembly spokesperson said: "It is welcome news that the three cancer networks in Wales all now have agreed guidance for prescribing Herceptin for early breast cancer treatment.
"Individual clinicians, working closely with their Local Health Boards, are best placed to decide in individual cases whether the drug is likely to be beneficial to the patient involved. That is a matter for their clinical judgement."