Anaheim-based Questcor Pharmaceuticals Inc.’s shares dropped today after it disclosed in a federal filing that it “became aware of a U.S. government investigation” involving its promotional practices for its sole drug.

The drug maker’s shares were down 33% in midday trading to a market value of $1.2 billion.

Questcor makes H.P. Acthar Gel, an injectable medication primarily sold for treating kidney disease nephrotic syndrome and for multiple sclerosis flare-ups. Acthar is approved for 19 different indications in total.

The company’s stock went on a roller-coaster ride last week after a short seller disclosed that insurer Aetna Inc. was going to limit coverage of Acthar to treatment of infantile spasms, a rare form of epilepsy.

That sent shares down 48% on Sept. 19. They rebounded partially the next day when company officials said that it didn’t expect Aetna’s decision to hurt its business and that it was in discussions with the insurer.