The 28-year-old, whose real name is Dwayne Michael Carter Jr., was in suburban St. Louis on Sunday for an outdoor performance at the Verizon Amphitheatre.
KTVI-TV reported that the rapper, accompanied by a large entourage, showed up Sunday at DePaul Health Center's emergency room.
Though the hospital would not confirm that Lil Wayne was at the hospital, the rapper wrote about it on his Twitter account.
Lil Wayne is about to release his new album, "The Carter IV." A previous album, "The Carter III," was the best-selling album of 2008 and the Grammy Award winner for best rap album of that year. In 2010, he served time in a New York jail after pleading guilty to a weapons charge.
The Associated Press contributed to this report.