How many Penidure LA 12 (benzathine penicillin G) injections should be administered?

Penidure LA 12 Dosing Recommendations

The number of Penidure LA 12 (benzathine penicillin G 1.2 million units) injections you need depends entirely on what condition is being treated—for early syphilis, give ONE injection; for late/latent syphilis of unknown duration, give THREE injections at weekly intervals; for rheumatic fever prevention, give ONE injection every 4 weeks (or every 3 weeks in high-risk situations) indefinitely.

Dosing by Indication

For Syphilis Treatment

Early Latent Syphilis (infection <1 year):

• Single dose of 2.4 million units IM (two 1.2 million unit injections) 1
• This is a one-time treatment

Late Latent Syphilis or Syphilis of Unknown Duration:

• Total of 7.2 million units administered as three doses of 2.4 million units IM each at 1-week intervals 1
• This means 6 injections of Penidure LA 12 total (2 injections per week × 3 weeks)
• Critical caveat: If any dose is missed, the entire sequence must be restarted in pregnant patients 1. For non-pregnant patients, intervals of 10-14 days between doses may be acceptable before restarting 1

Recent High-Quality Evidence (2025): The most recent randomized controlled trial demonstrated that one dose of 2.4 million units was noninferior to three doses for early syphilis, with 76% serologic response in both groups at 6 months 2. This challenges traditional three-dose regimens for early syphilis, though guidelines have not yet been updated to reflect this finding.

For Rheumatic Fever Prevention (Secondary Prophylaxis)

Standard Regimen:

• 1.2 million units IM every 4 weeks 3
• Continue indefinitely based on cardiac status:
  • With persistent valvular disease: Continue for 10 years after last episode OR until age 40 (whichever is longer), then consider lifelong 3
  • Without valvular disease: Continue until age 21 OR for 5 years after last episode (whichever is longer) 3

High-Risk Regimen:

• 1.2 million units IM every 3 weeks 3
• Recommended for:
  • Populations with high rheumatic fever incidence 3
  • Patients with recurrent acute rheumatic fever despite 4-week regimen 3

Evidence strongly supports the 3-week interval: Multiple studies demonstrate superior outcomes with 3-week versus 4-week dosing, with prophylaxis failure rates of 0.25 versus 1.29 per 100 patient-years respectively 4. The 3-week regimen maintains adequate serum penicillin levels (≥0.02 µg/mL) in 56% of samples at 21 days versus only 33% at 28 days with the 4-week regimen 4.

Critical Clinical Considerations

Before Treatment:

• All syphilis patients require HIV testing 1
• Patients with late latent syphilis, neurologic/ophthalmic symptoms, treatment failure, or HIV infection with late latent syphilis should have CSF examination to exclude neurosyphilis 1
• Children with syphilis need CSF examination to distinguish congenital from acquired syphilis 1

Pediatric Dosing:

• Early latent syphilis: 50,000 units/kg IM (up to adult dose of 2.4 million units) as single dose 1
• Late latent syphilis: 50,000 units/kg IM (up to 2.4 million units) × 3 doses at weekly intervals 1

Common Pitfalls:

• Do not confuse Penidure LA 12 (1.2 million units) with the full treatment dose of 2.4 million units—you need TWO injections of Penidure LA 12 to equal one treatment dose for syphilis
• Injection site pain and tenderness occur in 76-85% of patients 2
• The drug is highly viscous; ensure complete administration to avoid underdosing 5
• Pregnant women who miss any dose for late latent syphilis must restart the entire course 1

Follow-up for Syphilis:

• Quantitative nontreponemal tests at 6,12, and 24 months 1
• Re-treatment indicated if titers increase fourfold or fail to decline fourfold within 12-24 months 1