Penicillin G Benzathine for Syphilis: What the Injection Involves and How Many You Need

You got your result. Maybe you came in for a routine screen, maybe a partner reached out, or maybe something did not look right and you decided to get it checked. Either way, you are here now, and this is one of the most straightforward infections to treat. The antibiotic used, penicillin G benzathine, has been the drug of choice for syphilis for decades. One or three injections, depending on your stage, and the treatment is done.

A lot of tourists and expats who walk into Doctor Pattaya assume this will be complicated or drawn out. It rarely is. But there are things worth knowing before you roll up your sleeve: how the injection feels, what to expect in the hours after, and how to confirm it has actually worked. Let us get into it.

What Penicillin G Benzathine Is and Why It Works for Syphilis

Penicillin G benzathine is a slow-release form of penicillin given as a single intramuscular injection. Once it is in your muscle, it releases steadily over several days, keeping antibiotic levels in your blood high enough to kill the bacteria that cause syphilis.

That slow release is exactly what makes it ideal. The bacteria responsible for syphilis divide slowly, so they need sustained antibiotic exposure to be cleared. A tablet antibiotic that clears your system in hours would not do the job as reliably. The injection handles it in one visit, with no daily pill schedule to forget.

WHO, CDC, and Thai treatment guidelines all list it as first choice. No other antibiotic has the same depth of evidence behind it for syphilis.

How Many Penicillin G Benzathine Injections You Need, Stage by Stage

This is the question we hear most often. The answer depends on your stage, which a blood test confirms before we treat.

For primary syphilis, secondary syphilis, or early latent syphilis diagnosed within the past year, one injection of 2.4 million units is the standard course. A 2025 trial published in the New England Journal of Medicine confirmed that a single dose performs as well as three doses for early-stage disease. One visit. Done.

For late latent syphilis or syphilis of unknown duration, three weekly injections are needed, 2.4 million units each, totalling 7.2 million units across three weeks. Neurosyphilis, where the infection has reached the nervous system, is a different situation that requires intravenous treatment in hospital. That is rare, and we will refer you appropriately if the picture points that way.

Do not guess your stage. Get the blood test first. The doctor will tell you exactly what you need.

What the Penicillin G Benzathine Injection Actually Involves

This injection is not like a flu shot. Penicillin G benzathine is a thick liquid, and it goes deep into the muscle, usually the upper outer buttock. It takes a few seconds longer to administer than a standard injection, and the site can be sore for a day or two afterward.

To reduce discomfort, we can mix the penicillin with a small amount of lidocaine, a local anaesthetic. The evidence supports this approach, and it makes a real difference. Tell us if you are nervous about needles or have had painful injections before, and we will factor that in.

After the injection you can go about your day. There is no recovery period. Ice the area for a few minutes right after, and take paracetamol if the soreness bothers you. Avoid heavy exercise on that side for 24 hours.

The Jarisch-Herxheimer Reaction: Not an Allergy

This one catches people off guard, and it is worth knowing about before you leave the clinic.

In the hours after your first injection, you might develop a fever, chills, headache, and muscle aches. It usually starts within four to twelve hours and clears on its own within twenty-four hours. This is the Jarisch-Herxheimer reaction, an immune response to the bacteria dying off quickly. It is not a penicillin allergy. It is actually a sign the antibiotic is working.

Rest, stay hydrated, and take paracetamol. If you are staying at a hotel in Jomtien or Pratumnak and this hits you in the evening, do not panic. It is expected and self-limiting. Contact us if your fever is very high, symptoms last beyond twenty-four hours, or you develop difficulty breathing. Doctor Pattaya is open around the clock, and we can send a doctor to your hotel if needed.

Penicillin Allergy, HIV, and Pregnancy: What Changes

If you have a confirmed penicillin allergy, you still have options. For non-pregnant adults with early syphilis, doxycycline 100 mg twice daily for fourteen days is a recognised alternative. Ceftriaxone by injection is another option, though with a smaller evidence base.

If you are pregnant and allergic to penicillin, the situation is more complex. Penicillin is the only antibiotic with proven evidence for preventing the infection from passing to the baby. Desensitisation may be needed in that case, carried out in a hospital setting. We will refer you to the right facility if that applies.

HIV co-infection does not change the antibiotic choice or the number of injections for early syphilis. Follow-up monitoring may be more frequent. Always tell your doctor your HIV status and any medications you are taking, including PrEP or antiretrovirals, so we can put the right plan in place.

Follow-Up After Treatment: Confirming It Worked

Treatment is not the end of the process. A follow-up blood test confirms the syphilis has cleared. We use an RPR or VDRL titer, a blood marker that measures your body’s response to the infection. A significant drop in that level at three to six months after injection tells us the treatment worked.

Do not skip this step, even if you feel completely well. If it does not drop, or if it rises, something needs re-evaluating.

If you are a tourist who will be home by the time follow-up is due, you can get the blood test done locally and send us the results. We can review them and advise remotely. If you are based in Pattaya or Naklua, we will simply book you in. Our STD treatment service includes a structured follow-up plan so nothing gets missed.

Getting Penicillin G Benzathine Treatment at Doctor Pattaya

Walk-ins are welcome any time, day or night. No appointment needed, and nothing to explain to a receptionist before you see a doctor. Everything is confidential.

When you come in, we start with a blood test to confirm your stage. Rapid syphilis screening results come back quickly. Once we have your result and a brief history, including any allergy history and current medications, we draw up the injection. For straightforward cases, same-day treatment is the norm.

If you want comprehensive screening alongside treatment, we can run a full STD panel at the same visit, covering HIV, gonorrhoea, chlamydia, hepatitis B, and more. If you are within 72 hours of a potential HIV exposure, tell us right away. PEP is time-sensitive and needs to start as soon as possible.

If you cannot get to the clinic, Doctor Pattaya offers hotel visits across Pattaya, Jomtien, and Naklua. The doctor comes to you, completes the consultation, and administers the injection at your hotel. Discreet, practical, and fast. If you want to know more about how syphilis treatment works from start to finish, including how many injections different stages require, our STD treatment page covers it in full.