Late syphilis (late latent and tertiary syphilis of more than two years’ duration without evidence of treponemal infection) or unknown duration in pregnant women. If benzathine penicillin is not available, WHO suggests:

• procaine penicillin 1.2 million units intramuscularly once daily for 20 days.

In rare situations when benzathine or procaine penicillin cannot be used (e.g. due to confirmed penicillin allergy, which occurs in less than 3% of the population, and where penicillin desensitization is not possible) or are not available (e.g. due to stock-outs), WHO suggests using, with caution and enhanced follow-up:

• erythromycin 500 mg orally four times daily for 30 days.