Since the 1940s, syphilis has become a far less fatal infection, but a new shortage of penicillin has health agencies warning doctors to use the drugs wisely.

A shortage of an important penicillin injection has medical professionals being cautious about when they administer it.

Syphilis, which was a long death sentence before the discovery of penicillin in the 1940s, remains a problem, even in modern medicine.

Unprotected sex and tainted needles are major means of transmission, but the disease can always spread from mother to baby during pregnancy, labor, or nursing, according to the Centers for Disease Control and Prevention (CDC).

In 2014, 458 cases of congenital syphilis were reported. Overall that year, 63,450 new cases of syphilis were reported, a steady increase since 2000.

The majority of those new cases involved men who have sex with men, men in the West and in the South, and black men.

While the rates go up, the need for the antibiotic that can cure the infection is in greater demand.

And now that antibiotic is in short supply.

“Infectious diseases have not been conquered and are really only kept at bay through our vaccines and medications. When these countermeasures are absent or rendered obsolete through resistance, we risk allowing once controlled diseases to roar back,” Dr. Amesh A. Adalja, an infectious disease physician at the University of Pittsburgh Medical Center, told Healthline.

Read More: Get the Facts on Syphilis »

Benzylpenicillin, the active ingredient in the shots, is a narrow-spectrum antibiotic, meaning it’s only effective against a certain family of bacteria.

It’s on the World Health Organization’s list of essential medicines.

Three doses of long acting versions of benzylpenicillin, sold under the brand Bicillin L-A, is typically enough to kill the syphilis bacteria and prevent further damage.

Early detection is key, as the CDC notes, because while the drug can kill the bacteria, it cannot reverse the damage already done.

Without proper treatment, syphilis can cause neurological damage and is ultimately fatal.

Pfizer, the only manufacturer of the drug, sent out a letter dated April 27 warning of the impending interruptions in the supply chain due to a manufacturing delay. It affects the one, two, and four milliliter (mL) prefilled syringes.

“We recommend consideration of prioritizing where BICILLIN L-A is the only choice for the patient’s treatment,” the letter states. “Please consider alternative drugs where clinically applicable.”

The CDC is urging doctors to prioritize the use of Bicillin L-A for people with syphilis, namely pregnant women infected with or exposed to syphilis until normal supplies are available. Bicillin L-A is the only effective and safe treatment for pregnant women with syphilis.

Canadian health officials are acquiring Bicillin L-A from Pfizer’s Australian supply.

The shortage is expected to be short-lived. Pfizer says the backorder issues should be resolved in July.

Besides syphilis, the drug is approved to treat upper respiratory infections caused by streptococci as well as preventing recurrence of rheumatic fever.

But because of the shortage, the difficulties of treating syphilis may compound as hospitals hold onto the important medication while looking for suitable alternatives.

Adalja said the ailment is an infectious disease that has a long history and has continued to plague humans to this day.

“I, myself, have treated multiple cases of syphilis and there are pockets of very high levels of infection in certain parts of the U.S.,” he said.

Read More: Where Will The New Antibiotics Come From? »

According to the CDC, the high-incidence areas are in the South, where rates of STD are generally higher.

These states, coincidently, are also the ones that have repeatedly threatened to defund Planned Parenthood, which, among other things, offers education and services to lower STD rates.

Adalja says anytime there is a shortage of a vital medication it will have a deleterious effect as delays and second-line medications have to be used.

“Drug supply shocks are a major concern, especially when only an inferior substitute is available,” he said.

While Bicillin L-A can be used for other purposes, the CDC’s request that doctors prioritize its use for syphilis is yet another example of how doctors and others need to exercise caution and only use antibiotics for limited purposes.

This is true more than ever as few new antibiotics are being discovered and developed while drug-resistant bacteria continue to adapt to the limited supply of antibiotics currently available.

A recent study found that up to 40 percent of prescriptions for antibiotics — namely for asthma or upper respiratory infections — were prescribed for conditions that they could not cure. Instead, they created unwanted side effects and helped propel antibiotic resistance.

It also makes necessary drugs harder to find when they’re needed most; leaving people open to potentially lethal infections that would otherwise be easily curable.

“Antibiotics are a very precious resource and should not be squandered through inappropriate use. When a drug is in limited supply it is very important to only use it when truly necessary,” Adalja said.

Read More: Weakening Antibiotics Could Result in 6,300 More Infection-Related Deaths Per Year »