March 18 (Reuters) - Biogen Idec Inc said the U.S. Food and Drug Administration extended the review process for the company's multiple sclerosis drug by three months to evaluate the application.

Biogen, which was expecting to launch the drug, Plegridy, by mid-2014, said the FDA did not ask for additional studies.

Plegridy, is an injectable drug designed to reduce the dosing schedule of standard interferon drugs such as Biogen's own Avonex, which are typically dosed at least once a week.

Interferon is hard to tolerate as it leads to flu-like symptoms, prompting patients to delay or discontinue treatment.

Analysts say the market for such interferon-based treatments will shrink over the next decade as newer generation products enter the fray.

Multiple sclerosis is a chronic condition that occurs when the body's immune system mistakenly attacks and destroys the protective sheath surrounding nerve cells in the brain, optic nerve or spinal cord.

Symptoms may include loss of balance, difficulty moving arms and legs, weakness, numbness and blindness.

Biogen shares closed at $345.60 on Monday on the Nasdaq.