Pfizer is facing a shortage of penicillin due to increased demand and more diagnoses of syphilis, the company said in a letter to its customers this week.

The company said there is an "impending stock out situation" for select Bicillin L-A and Bicillin C-R prefilled syringes, Pfizer's brand name of injectable penicillin.

Prefilled Bicillin L-A pediatric syringes are expected to begin running low by the end of June, while all Bicillin C-R syringes could begin diminishing in the third quarter of this year.

Inventory is predicted to start recovering in the second and third quarters of 2024, Pfizer said.

Penicillin is an antibiotic used to treat various types of infections, including sexually transmitted diseases, such as syphilis and pneumonia.

Syphilis rates were at a historic low from 2000 to 2001, but have increased every year since, with an increase of 28.6% from 2020 to 2021, which was likely exacerbated by an interruption in treatment and prevention due to the COVID-19 pandemic, the Centers for Disease Control and Prevention said.

The scarcity of penicillin comes amid a long-term shortage of prescription drugs the U.S. has been experiencing for the last decade, usually as a result of disruptions in the supply chain, like the one caused by the pandemic, Marta Wosińska, a visiting fellow at The Brookings Institution, told NPR earlier this year.

Other prescriptions, such as Adderall and children's flu medication have also been in short supply.