For 15 years, Susan Cohn-Child has injected herself with a multiple sclerosis drug at least once a week to keep the disease in check. But Cohn-Child and thousands of other MS patients may soon be able to replace their needles with a pill being developed by Biogen Idec Inc. Following promising trial results, the Weston biotechnology company yesterday filed an application with the US Food and Drug Administration seeking approval to sell the oral treatment, now known as BG-12. Most treatments for the neurological disorder, which affects about 400,000 people in the United States, involve regular injections or intravenous infusions.