Merck ( MRK) said Thursday that it had received regulatory approval for its migraine headache treatment Vioxx. Merck said results from studies demonstrated that a single dose of Vioxx 25 mg or 50 mg provided significant migraine headache pain relief within two hours and also reduced the incidence of migraine symptoms, such as sensitivity to light. Some 28 million people in the U.S. suffer from the effects of migraines, the company said in a statement. Vioxx had already been granted Food and Drug Administration approval for treating pain related to rheumatoid arthritis and surgery. Whitehouse Station, N.J.-based Merck reports first-quarter earnings on April 22. Shares rose 69 cents, or 1.6%, to $44.88, but remain near their 52-week low of $40.57.