Penicillin kills syphilis. Specifically, a single injection of long-acting penicillin G benzathine (2.4 million units) cures primary, secondary, and early latent syphilis in most adults. This has been the standard treatment since the 1950s, and no other antibiotic works as reliably.
How Penicillin Destroys the Bacteria
Syphilis is caused by a spiral-shaped bacterium called Treponema pallidum. Like most bacteria, it relies on a rigid outer wall to hold its shape and protect it from bursting. The wall is made of a mesh-like material that the bacterium constantly rebuilds as it grows and divides.
Penicillin locks onto the enzymes responsible for stitching that wall together. Once those enzymes are permanently disabled, the bacterium can no longer repair or expand its protective shell. Meanwhile, other natural processes keep breaking the wall down. Water rushes into the weakened cell through osmotic pressure, and fragments of the collapsing wall trigger additional enzymes that tear it apart from within. The bacterium essentially ruptures and dies.
This is why penicillin works so well against syphilis: Treponema pallidum divides slowly (roughly once every 30 hours), and the long-acting form of penicillin stays in your body long enough to catch the bacteria during their vulnerable growth phase.
What Treatment Looks Like for Each Stage
For early syphilis (primary, secondary, or early latent), treatment is a single shot in the muscle. One visit, one injection. Additional doses of penicillin or other antibiotics don’t improve the outcome for early-stage infections, regardless of HIV status.
Late latent syphilis, meaning an infection that has been present for more than a year without symptoms, requires three injections given one week apart.
Neurosyphilis, where the infection has reached the brain or spinal cord, is more intensive. It requires intravenous penicillin for 10 to 14 days. This means daily hospital or clinic visits for IV treatment, since the antibiotic needs to reach high enough concentrations in the central nervous system to clear the bacteria there.
Newborns diagnosed with congenital syphilis receive intravenous penicillin for 10 days. The dosing is weight-based and adjusted during the first week of life.
Options if You’re Allergic to Penicillin
For people with a confirmed penicillin allergy, doxycycline taken twice daily for 14 days is the primary alternative for early syphilis. Ceftriaxone, given by injection, is another option, particularly for neurosyphilis, where it’s administered daily for 10 to 14 days.
Azithromycin was once considered a promising oral alternative, but it’s no longer appropriate in most parts of the world. More than 80% to 85% of syphilis strains in many regions carry a genetic mutation that makes them resistant to azithromycin and related antibiotics. In some areas, like Shanghai, resistance rates reach 97%. This level of resistance makes azithromycin too unreliable to use.
Can the Bacteria Become Resistant to Penicillin?
For decades, the assumption was that Treponema pallidum simply couldn’t develop penicillin resistance. That confidence has started to shift. A 2025 study published in Nature demonstrated that the bacterium is genetically capable of developing partial resistance to penicillin and ceftriaxone through a specific mutation. Researchers found this mutation already present in the majority of circulating syphilis strains worldwide.
The key word is “partial.” The level of resistance discovered so far is not enough to cause treatment failure at current doses. Penicillin still works. But the finding raises concern that full resistance could eventually emerge, especially as syphilis rates climb globally. For now, there is no documented case of a syphilis strain that is fully resistant to penicillin in a clinical setting. Reports of treatment failure do exist, though the causes remain debated.
What to Expect After Treatment
Within a few hours of your first penicillin injection, you may experience what’s known as the Jarisch-Herxheimer reaction. This happens when large numbers of bacteria die quickly, releasing fragments that trigger a temporary inflammatory response. Symptoms typically start within 2 hours and can include fever, chills, muscle aches, headache, nausea, and a temporary flare-up of any existing skin sores. It can feel alarming, but it’s a sign the antibiotic is working. Symptoms almost always resolve on their own within 12 to 24 hours.
After treatment, you’ll need blood tests to confirm the infection is clearing. The standard approach measures antibody levels in your blood at regular intervals. For HIV-negative people, follow-up testing is recommended at 6 and 12 months after treatment. For those living with HIV, testing is more frequent: at 3, 6, 9, 12, and 24 months. A successful response means your antibody levels drop by at least fourfold (for example, from 1:32 down to 1:8 or lower). Some people eventually test completely nonreactive, while others maintain low-level positive results indefinitely.
What Penicillin Can’t Undo
Penicillin kills the bacteria at every stage of syphilis, but it cannot reverse damage that has already occurred. This matters most in tertiary syphilis, which develops 10 to 30 years after an untreated infection and can affect the heart, blood vessels, brain, and nervous system. Treating at this stage stops the infection from progressing further, but organ damage, nerve injury, and cardiovascular changes that have already taken place may be permanent.
This is why timing matters so much. In its early stages, syphilis is one of the most curable infections in medicine. A single shot ends it. But every month or year of delay gives the bacterium more time to cause harm that no antibiotic can fix.