(The Hill) – The ongoing drug shortages impacting the U.S. are now threatening to exhaust two commonly used forms of penicillin, Pfizer said in a warning to the Food and Drug Administration (FDA) on Monday.

Pfizer alerted the FDA that there is “a limited supply and impending stock out” of Bicillin L-A and Bicillin C-R, two of its injectable penicillin products.

Bicillin L-A is used to treat strep infections in the throat, nose or skin; syphilis or syphilis-like infections; and also to prevent problems stemming from strep. Bicillin C-R is also used to treat strep infections in the throat, nose, skin or soft tissues as well as scarlet fever, pneumonia and ear infections.

Bicillin injections are colloquially referred to as the “peanut butter shot” in the military due to the color of the drug. The injection is widely known as being particularly painful. Neither of these drugs are available in lower-cost generic forms.

Pfizer cited a “complex combination of factors” for the shortage, including an increase in demand and a rise in syphilis rates.

The company estimated that supplies of Bicillin C-R would be depleted around the third quarter of this year, while pediatric pre-filled syringes of Bicillin L-A would be depleted even earlier.

This warning comes amid shortages of several other crucial medications. Cancer centers in the U.S. are reporting shortages of two key chemotherapy drugs, while the Adderall shortage that was announced late last year persists.