A new shortage of a type of penicillin crucial to the fight against syphilis is alarming infectious disease experts, who warn that a protracted scarcity of the drug could worsen the U.S. epidemic of the sexually transmitted infection.

The shortage, announced by the drugmaker Pfizer in a letter last month, involves Bicillin L-A, a long-acting injectable antibiotic also known as penicillin G benzathine. The company cited significant increases in demand because of the rising rate of syphilis infections, as well as Bicillin’s recent use as an alternative to amoxicillin, another antibiotic that has periodically been scarce and is prescribed for more general infections like strep throat.

Steven Danehy, a spokesman for Pfizer, said it would likely take about a year for the company to ramp up production by 50 percent at its plant in Rochester, Mich., and ultimately manufacture enough Bicillin to meet demand and shore up reserves.

Syphilis has been on the rise in the United States since 2000, reaching 176,713 cases in 2021, which was an increase of nearly 75 percent since 2017, according to the Centers for Disease Control and Prevention.

Congenital syphilis tripled during that four-year period, to 2,855 cases, including 220 stillbirths or infant deaths. Rates are highest among the infants of Native American, Native Hawaiian, Pacific Islander and Black mothers.

Bicillin is the only recommended treatment for pregnant women who are infected, and is very effective at preventing transmission to the fetus if provided early enough. Congenital syphilis has a high fatality rate, and can otherwise cause preterm birth and severe birth defects.

“It worries me that these moms may not have access to lifesaving medication,” Dr. Anita Henderson, a pediatrician in Hattiesburg, Miss., said. The state had seen large increases in the rate of congenital syphilis over the last five years, she said.

Among adult syphilis cases, nearly one-fourth are in women; just under a third are in men who have sex only with men; and about one-fifth are in men who only have sex with women.

The infection can cause sores and a rash and, if left untreated, can seriously damage the internal organs, nervous system, eyes and ears, and can be fatal.

Pfizer also warned that its supply of a rarely used pediatric version of Bicillin would soon run out because the company had begun using that drug’s production line to increase the adult formula. Doctors turned to it in the last year in lieu of amoxicillin during an increase in the number of strep throat cases.

Bicillin is also used to manage rheumatic heart disease and rheumatic fever, which are particular health risks, albeit uncommon, for children. Multiple antibiotic alternatives are available for these conditions, according to Dr. Meg Doherty, director of global H.I.V., hepatitis and sexually transmitted infections programs at the World Health Organization.

To ward off bacterial infections, military recruits receive Bicillin during boot camp, where the drug is known as the “peanut butter shot” because of its color and consistency. According to Dr. Ryan C. Maves, a professor of infectious diseases at Wake Forest University School of Medicine, recruits otherwise face a high risk of invasive streptococcal infection.

Alternatives to Bicillin for pregnant women are under development and review but are years away from becoming available to them, said Dr. Jeffrey Klausner, an infectious disease expert at the University of Southern California. He urged the Biden administration to pay Pfizer for some 500,000 doses to encourage production.

The Bicillin shortfall is but one element of a widespread drug-shortage crisis that has left doctors and pharmacists scrambling for vital therapeutic staples and forced them to ration treatments like chemotherapy. A recent Senate report also characterized the supply problems as a threat to national security.

Most drug companies have not been particularly keen on developing antibiotics, in part because the profit margin for this class of drugs is typically far lower than the next blockbuster drug that could be worth billions of dollars.

A bipartisan group in Congress recently reintroduced the $6 billion Pasteur Act, a Netflix-like subscription model that would act as a financial incentive for research and development by pharmaceutical companies.While the legislation could address drug shortages, its main goal is to combat the global threat of drug-resistant pathogens.

David Harvey, executive director of the National Coalition of S.T.D. Directors, a trade association for public health associations, said rates of syphilis, chlamydia and gonorrhea are all surging “in part due to a public health landscape that is stretched dangerously thin, resulting in a lack of S.T.I. prevention, testing, and treatment.”

He and others criticized Pfizer for inadequate production of the drug given the decades-long rising trajectory of syphilis infections. But Pfizer’s spokesman, Mr. Danehy, said the company had invested $38 million in its Michigan plant to improve manufacturing after a previous shortage of Bicillin in 2017.

Mr. Harvey also denounced the Biden administration for agreeing in the debt ceiling deal to slash $400 million from the C.D.C.’s budget for S.T.I. prevention.

To stretch the Bicillin supply, the C.D.C. recommends that doctors give preference to pregnant patients and infected or exposed infants. Other patients should instead be prescribed doxycycline for two to four weeks, depending on the disease stage. But experts expressed worry that such individuals, including the partners of pregnant women, might have trouble sticking to the twice-daily pill regimen, potentially compromising its effectiveness.

Eric Tichy, division chair of supply chain management at Mayo Clinic in Rochester, Minn., said Pfizer likely stands alone in producing Bicillin for the U.S. market because of the considerable complexity and expense of manufacturing the drug.

But other experts objected to Pfizer’s pricing practices. “Here’s a prime example of why leaving public health to the free market can be disastrous,” Tim Horn, director of medication access at the National Alliance of State and Territorial AIDS Directors, an advocacy group, said in an email.

“Since 2013, the price of Bicillin L-A has increased an astonishing 275 percent,” Mr. Horn said.

Mr. Danehy said the list price for a 4-milliliter Bicillin L-A syringe is $470, and that the company adjusted prices to ensure proper, quality supply.

While many health care organizations and clinics are able to secure discounts, some frontline independent clinics are paying top dollar for the antibiotic.

Dr. Phyllis Ritchie, who runs a free S.T.I. clinic largely serving gay men in Palm Springs, Calif., said the cost of a 10-pack of shots of Bicillin had risen to $6,500 from $4,000 two years ago. With the clinic using about 15 to 20 of the 10-packs annually, its $225,000 yearly budget can no longer withstand the financial strain, she said.

When she first began practicing medicine in the mid-1990s, Dr. Ritchie recalled, a 10-pack cost under $300.

“It’s a crisis,” she said.