"Both were tested for steroids and diuretics in mid-October and passed," Kizer said.
Carwin and Nelson were opposing coaches on Season 16 of "The Ultimate Fighter." They have directed harsh words toward one another for several months.
Their fight headlines TUF 16 Finale at Hard Rock Hotel & Casino in Las Vegas.
A former UFC interim heavyweight champion, Carwin (12-2) has not fought since June 2011, when he came out on the short end of a lopsided unanimous decision to Junior dos Santos.
Carwin suffered a back injury during training, requiring surgery.
Nelson (17-7) is a former TUF heavyweight winner and International Fight League champion. On March 26, in his most recent fight, Nelson knocked out Dave Herman in the first round.