WASHINGTON, April 20 — The Food and Drug Administration said Thursday that "no sound scientific studies" supported the medical use of marijuana, contradicting a 1999 review by a panel of highly regarded scientists. The announcement inserts the health agency into yet another fierce political fight. Susan Bro, an agency spokeswoman, said Thursday's statement resulted from a past combined review by federal drug enforcement, regulatory and research agencies that concluded "smoked marijuana has no currently accepted or proven medical use in the United States and is not an approved medical treatment."