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We studied antibiotic efficacy in 113 patients with erythema chronicum migrans, the first manifestation of
Lyme disease. Erythema chronicum migrans and its associated symptoms resolved faster in patients given penicillin or tetracycline (median duration, 4 and 2 days, respectively) than in untreated patients (10 days; P less than 0.001 and P = 0.005, respectively). Erythromycin had no significant effect. Although the frequency of subsequent neurologic and cardiac abnormalities was similar in all four groups, significantly fewer patients given penicillin developed arthritis than did untreated patients (P = 0.001). Among 15 patients with arthritis who were followed for at least 29 months, the total duration of joint involvement was shorter in penicillin-treated patients (median, 4 weeks) than in untreated patients (17 weeks; P = 0.019). Although the clinical manifestations of the
disease may fluctuate in frequency from year to year and influence apparent antibiotic effect, we conclude that penicillin therapy shortens the duration of erythema chronicum migrans and may prevent or attenuate subsequent arthritis.