What is the recommended intramuscular dosage of benzathine penicillin G (BPG) for early syphilis, late latent syphilis, and secondary prophylaxis of rheumatic fever?

Benzathine Penicillin G Dosage

For early syphilis (primary, secondary, or early latent), administer benzathine penicillin G 2.4 million units IM as a single dose; for late latent syphilis or syphilis of unknown duration, administer 2.4 million units IM weekly for three consecutive weeks (total 7.2 million units); for secondary prophylaxis of rheumatic fever, administer 1.2 million units IM every 4 weeks (or every 3 weeks in high-risk populations or those with recurrent episodes). 1, 2, 3

Syphilis Treatment Dosing

Adults

Early Syphilis (Primary, Secondary, or Early Latent <1 year duration):

• Single dose: 2.4 million units IM once 1, 3
• This single-dose regimen is supported by the most recent high-quality evidence showing noninferiority to three-dose regimens, with 76% serologic response at 6 months 4
• The 2025 NEJM trial definitively demonstrated that one dose is noninferior to three doses for early syphilis, regardless of HIV status 4

Late Latent Syphilis or Latent Syphilis of Unknown Duration:

• Three-dose regimen: 2.4 million units IM weekly for 3 consecutive weeks (total 7.2 million units) 1, 3
• This extended regimen is necessary because treatment aims to prevent progression of late complications rather than affect transmission 1

Pediatric Dosing

Early Latent Syphilis:

• 50,000 units/kg IM (maximum 2.4 million units) as a single dose 1

Late Latent Syphilis or Unknown Duration:

• 50,000 units/kg IM weekly for 3 weeks (total 150,000 units/kg, maximum 7.2 million units total) 1

Rheumatic Fever Secondary Prophylaxis

Standard Regimen:

• 1.2 million units IM every 4 weeks 2, 3
• This is the recommended interval for most patients in the United States 2

Intensive Regimen (High-Risk Situations):

• 1.2 million units IM every 3 weeks 2
• Use this interval for:
  • Populations with particularly high rheumatic fever incidence 2
  • Patients with recurrent acute rheumatic fever despite adherence to every-4-week regimen 2
  • Evidence shows superior outcomes: 0.25 vs 1.29 recurrences per 100 patient-years (3-week vs 4-week regimen) 5
  • Adequate serum penicillin levels (≥0.02 mcg/mL) maintained in 56% at 21 days vs 33% at 28 days 5

Duration of Prophylaxis

The duration depends on cardiac involvement:

With Persistent Valvular Disease:

• Continue for 10 years after last episode OR until age 40 (whichever is longer)
• Consider lifelong prophylaxis for high-risk patients after age 40 2
• Continue even after valve surgery, including prosthetic valve replacement 2

Without Persistent Valvular Disease:

• Continue until age 21 OR for 5 years after last attack (whichever is longer) 2

Administration Technique

Critical Safety Points:

• Administer by DEEP intramuscular injection ONLY 3
• Never inject intravenously, intra-arterially, or near nerves 3
• Preferred sites:
  • Adults: Upper outer quadrant of buttock (dorsogluteal) or ventrogluteal 3
  • Neonates/infants/small children: Midlateral aspect of thigh 3
  • Avoid anterolateral thigh due to adverse effects 3
• Inject at a slow, steady rate to prevent needle blockage from high concentration of suspended material 3
• Rotate injection sites when doses are repeated 3

Important Clinical Considerations

Before Treatment:

• All patients with latent syphilis require careful mucosal surface examination (oral cavity, perineum, foreskin) to exclude internal lesions 1
• All syphilis patients should be tested for HIV 1
• Consider CSF examination if:
  • Neurologic/ophthalmic symptoms present 1
  • Evidence of tertiary syphilis 1
  • Treatment failure 1
  • HIV infection with late latent syphilis 1
  • Nontreponemal titer >1:32 (some specialists recommend) 1

Missed Doses:

• For late syphilis treatment: intervals of 10-14 days between doses may be acceptable before restarting 1
• Exception: Pregnant patients must repeat the full course if any dose is missed 1

Pain Management:

• Both single 2.4 million unit doses and divided 1.2 million unit doses (in each buttock) are equally well tolerated 6
• Most patients experience local injection-site pain and tenderness (76-85%) 4

Adverse Events:

• Hypersensitivity reactions occur more frequently with antibiotics vs no treatment 7
• Local reactions are common 7
• Life-threatening allergic reactions are rare with long-term prophylaxis 2
• Anaphylaxis and sciatic nerve injury remain very rare 7