Zurich: Novartis aims to give away 100 doses of its $US2.1 million-per-patient ($3.05 million) Zolgensma for spinal muscular atrophy (SMA) in 2020 in a free-drug program that one patient group worried was a "health lottery" that could neglect some babies.

Starting January 2, Novartis' AveXis unit which developed Zolgensma will set aside 50 doses of the world's costliest single-dose treatment through June for babies under 2-years-old, Novartis said on Thursday, with up to 100 total doses to be distributed through 2020.

Spinal muscular atrophy affects the motor neurons of the spinal cord, causing muscle weakness and wasting. Credit:Louise Kennerley

The program would apply to countries where the medicine is not yet approved for the rare genetic disorder that affects only about 1 in 10,000 live births, but which can lead to death and profound physical disabilities.

Zolgensma, with sales of $US175 million through September, won US approval in May and has been touted as potentially curative for babies treated before symptoms begin. It is not available in Australia.