Shares of Mylan were up more than 18 percent intraday Wednesday, on pace for its best day since Oct. 13, 2008. Teva was down about 14 percent, on pace for its worse day since Aug. 3.
Mylan, whose stock was down 14 percent this year as of Tuesday's close, said that shipping was imminent for its Glatiramer Acetate 40 mg/mL for 3-times-a-week injection and 20 mg/mL for once-daily injection.
MS is a chronic disease of the central nervous system and afflicts an estimated 400,000 Americans.
The approvals "mark another significant milestone for our company, reinforce our proven capabilities in bringing complex and difficult-to-manufacture products to market, and further our commitment to providing access to high-quality medicines," Mylan CEO Heather Bresch said in a press release.
The FDA on Monday said it would introduce new measures to speed up bringing generic versions of complex drugs to the market. The moves came as the agency attempts to deal with the rising cost of drugs in the U.S.
In a note to clients on Wednesday, Mizuho Securities analyst Irina Koffler said she expects Mylan "to rally because Copaxone approval has eluded it for years, and the news validates the company's ability to execute on complex generics."
Koffler has a buy rating on the stock with a price target of $37 over a 12-month time frame.
—Reuters contributed to this report.