Alexander M. Haig Jr., the former secretary of state who is a candidate for the Republican presidential nomination, was admitted to Fairfax Hospital yesterday after suffering a severe allergic reaction to medication he had taken for a tooth abscess, hospital officials said.

Haig, 63, was listed in good condition and was expected to stay at the hospital overnight for observation, hospital spokeswoman Mary Stuart Rizk said. "He's doing just fine," Rizk said.

Haig complained of a toothache while campaigning in New Hampshire yesterday morning and went to Walter Reed Army Medical Center for treatment after returning to Washington. Later, while Haig was at his home in McLean, he experienced an "anaphylactic reaction" to the combination of Tylenol and penicillin, hospital officials said.

His face flushed, his tongue swelled and "he became faint or lightheaded," said Dr. Thomas Barsanti, an emergency room physician.

An ambulance took Haig to the emergency room about 5 p.m. On the way, paramedics gave him epinephrine and an antihistimine, and the effects were quickly reversed, Barsanti said.

"If he had not received treatment, there is a potential that he could have been in serious danger," Barsanti said. Haig was being kept at the hospital, in the cardiac care unit, because anaphylactic reactions can recur, Barsanti said.