It has been four weeks since Margaret Dallman had the most recent injection into her eyes and already her vision is starting to distort.

But doctors can no longer offer Mrs Dallman a $200 Medicare rebate for the bimonthly injections, which have dramatically repaired her failing vision, following a policy change that doctors say could hurt thousands of vulnerable older Australians.

New rules restrict the subsidy to drugs specifically approved as eye injections, leaving Mrs Dallman and thousands of others with macular degeneration - the most common form of age-related blindness - to bear the full cost of the procedure for injections of Avastin, which does not have the approval but is in widespread use.

The rule, issued without notice earlier this month, also applies to injections of antibiotics used to treat or prevent infections after cataract surgery and cortisone injections for diabetic retinopathy. It leaves only injections of Lucentis - an approved treatment for macular degeneration - eligible for Medicare funding.

But some patients, including Mrs Dallman, have been told the precise nature of their condition means they do not qualify for subsidised access to Lucentis, which otherwise costs $2000, and that the cheaper Avastin - about $100 - is more suitable, even though it has not been through formal trials.

The injections mean Mrs Dallman can keep driving and living independently. "When it's coming towards the end of the time, the blurriness and distortion come back," said the 70-year-old, from Dunedoo in western NSW.

With her sister's help, Mrs Dallman may try to pay for one or two more injections at about $500 each, but, "by Christmas time it's going to be very hard," she said. "If it's a cost thing, aren't they better to have me at home than having me with a white stick somewhere?"

However, costs have blown out. Medicare statistics show the Federal Government paid $16 million for the eye-injection procedure last financial year, versus $1 million three years previously.