The penicillin shortage began in April. The FDA said it was caused by increases in cases of both syphilis and strep throat. Because a common antibiotic used to treat strep was in short supply this winter, patients were prescribed penicillin as an alternative, driving up demand for the drug.

That created an acute problem at a time when syphilis cases were rising precipitously. Rates of syphilis in 2021 were the highest since 1990, according to the most updated government data. In 2021, at least 176,000 cases of syphilis were reported, 32% more than the year before. The infection disproportionately impacts gay and bisexual men.