Benzylpenicillin: What is it? Side Effects, Dosage, Interactions and Presentations

It is a penicillin antibiotic that is administered by deep intramuscular injection.

Benzylpenicillin, administered as penicillin G benzathine,  kills susceptible bacteria by inhibiting the synthesis or formation of the bacterial cell wall. The final result of the inhibition of bacterial cell synthesis is cell lysis and the death of susceptible bacteria.

Therefore, Benzylpenicillin has poor solubility and is released very slowly from the injection site. In the blood, penicillin G benzathine is broken down into penicillin G.

The penicillin G benzathine formulation is released and absorbed slowly to allow a more prolonged action (the drug works gradually over a more extended period).

Available brands

  • LA Bicilina.

Generic: Not available in the US UU

Side effects of Benzylpenicillin

Side effects associated with treatment with Benzylpenicillin are:

  • Hypersensitivity or allergic reactions.
  • Stomachache.
  • Neuropathy.
  • Heart problems.
  • Dizziness.
  • Fatigue.
  • Headache.
  • Nervousness.
  • Convulsions
  • Weakness.
  • Sexual dysfunction
  • Reactions at the injection site.
  • Blurry vision.
  • Renal problems.
  • Others.


In streptococcal upper respiratory tract infections: adult patients are given a single injection of 1,200,000 units; older pediatric patients are given a single injection of 900,000 units; Babies and pediatric patients <60 Ibs are administered from 300,000 to 600.00 units.


For the treatment of primary, secondary, and latent syphilis: the recommended dose is 2,400,000 units as a single dose.

For the treatment of tertiary neurosyphilis: the recommended dose is 2,400,000 every seven days for three doses.

For the treatment of congenital syphilis in patients younger than two years: the recommended dose is 50,000 units/kg.

Rheumatic fever and glomerulonephritis prevention after an acute attack: Benzylpenicillin can be administered in 1,200,000 units once a month or 600,000 units every two weeks.

Interactions of Benzylpenicillin

Penicillin G benzathine may decrease renal tubular secretion (elimination through the kidneys) of methotrexate (Trexall, Rheumatrex), causing an increase in blood levels of methotrexate. Patients in combination therapy should be monitored closely for side effects.

It can interfere with the body’s response to the typhoid vaccine (Vivotif Berna vaccine). The general recommendation is to wait 24 hours or more from administering the last dose of the antibiotic before administering the vaccine.

Penicillin G benzathine may decrease the effectiveness of oral contraceptives (birth control pills). Patients may need to use a backup method of contraception while receiving antibiotic treatment.

Coadministration of penicillin G benzathine with warfarin (Coumadin) may increase the risk of bleeding. Antibiotics can inhibit the synthesis of vitamin K and warfarin is an antagonist of vitamin K (which also inhibits the action of vitamin K).

Patients on combination therapy should be monitored closely for signs or symptoms of bleeding.

Pregnancy and lactation

Benzylpenicillin is excreted in human milk. Due to the lack of safety data, benzathine penicillin G should be used with caution in breastfeeding women.

Presentations of Benzylpenicillin

Injectable penicillin G benzatine suspension: 1 ml (600,000 units per syringe), 2 ml (1,200,000 units per syringe) and 4 ml (2,400,000 units per syringe).


The suspension should be stored in a refrigerator, between 2 C and 8 C (36 F and 86 F).


Injectable suspension of penicillin G benzathine is an antibiotic prescribed to treat upper respiratory tract infection and sexually transmitted diseases such as syphilis. It is also prescribed to prevent rheumatic fever (chorea).

Side effects, drug interactions, dosage, storage, and safety information for pregnancy should be reviewed before using this medication.