Shares of Anaheim drug maker Questcor Pharmaceuticals Inc. plunged today after an online newsletter run by a short seller said that insurer Aetna Inc. is going to limit coverage of its sole drug.

Investors sent Questcor’s stock down about 37% in midday New York trading to a market value of $2 billion.

Questcor makes H.P. Acthar Gel, an injectable drug that is approved for 19 uses. The drug maker primarily sells it for kidney disorder nephrotic syndrome and for treating multiple sclerosis flare-ups.

Hartford, Conn.-based Aetna recently issued a policy bulletin that reversed its previous coverage of Acthar and said that clinical research only supported one use—for infantile spasms, a very rare condition. Questcor markets the drug for several other uses.

Citron Research, a longtime Questcor critic and a short seller of its shares, published an article on Aetna’s policy bulletin on its website. Citron has repeatedly asserted that Acthar’s price, which runs roughly $40,000 to $100,000 per treatment, can’t be backed up by superiority claims.