Pfizer has warned that a drug used to treat syphilis and other bacterial infections in children could run out by the end of June as a spike in infections has put pressure on an already short supply of the antibiotic.

Supply of the drug, Bicillin L-A, is expected to be exhausted by the end of this quarter, the company said in a letter to the U.S. health regulator dated Monday. Pfizer's penicillin products have been in shortage since April.

Another similar penicillin product, Bicillin C-R, which is used to treat infections related to the upper-respiratory tract, is expected to run out of supply in the third quarter, Pfizer said.

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Pfizer's warning comes amid a shortage of the widely used antibiotic amoxicillin since October.

Cases of congenital syphilis, when the infection passes to babies during pregnancy, have more than tripled in recent years, with more than 2,000 cases reported in 2021, according to latest government data.

To meet the increased demand, Pfizer said it has prioritized the manufacturing capacity for Bicillin L-A.