OMACILLIN 500/MG CAP 20/CAP

Indications

Approved Uses

Susceptible bacterial infections including: ear, nose and throat infections (e.g., otitis media, sinusitis, pharyngitis/tonsillitis), lower respiratory tract infections, genitourinary tract infections, and skin/skin structure infections; and eradication of Helicobacter pylori as part of combination therapy to reduce duodenal ulcer recurrence.

Dosage & Administration

Initial Dose

500mg every 8 hours

Children's Dosage

Children (>3 months and <40kg): 20-45 mg/kg/day in divided doses every 8-12 hours. For severe infections, 40-45 mg/kg/day in divided doses every 8-12 hours.

Side Effects

Common Side Effects

Diarrhea, nausea, vomiting, rash, and headache; candidiasis (e.g., vaginal yeast infection/oral thrush) can occur.

Side Effect Frequency

Common (≥1% to <10%): diarrhea, nausea, vomiting, and rash/urticaria. Uncommon (<1%): pruritus, candidiasis, and other hypersensitivity reactions; rare/very rare: severe cutaneous reactions, C. difficile-associated colitis, hematologic abnormalities, hepatitis/cholestatic jaundice, interstitial nephritis, and seizures (more likely with renal impairment/high doses).

Safety & Warnings

Age Restriction

No fixed minimum age restriction; neonates/infants ≤3 months require reduced dosing (e.g., max ~30 mg/kg/day divided q12h) due to immature renal clearance.

Drug Interactions

Drug Interactions

Key interactions: probenecid (↑ amoxicillin levels), warfarin/other oral anticoagulants (↑ INR/bleeding risk), methotrexate (↓ clearance/↑ toxicity), allopurinol (↑ rash risk), possible reduced reliability of combined oral contraceptives (advise backup if vomiting/diarrhea), and avoid relying on bacteriostatic agents (e.g., tetracyclines) for synergy as they may antagonize effect in some settings.

Interaction Severity

MAJOR: Methotrexate (reduced clearance/increased toxicity risk). MODERATE: Warfarin (may increase INR/bleeding risk), Probenecid (increases amoxicillin levels), Allopurinol (increased rash risk). MINOR/COUNSELING: Hormonal contraceptives (evidence of reduced efficacy is limited; advise backup if vomiting/diarrhea or per local policy).

Food Interaction

No clinically significant food interaction; may be taken with or without food (food may improve GI tolerability).

Special Populations

Children

Children (>3 months and <40kg): 20-45 mg/kg/day in divided doses every 8-12 hours. For severe infections, 40-45 mg/kg/day in divided doses every 8-12 hours.

Kidney Impairment

CrCl 10-30 ml/min: 250-500mg every 12 hours. CrCl <10 ml/min: 250-500mg every 24 hours. Hemodialysis patients: 250-500mg every 24 hours with an additional dose during and after dialysis

Storage & Patient Advice

Stopping the Medicine

Complete the full prescribed course. Do not stop early, even if symptoms improve, to prevent the return of the infection and development of resistance.

Overdose

Overdose may cause GI upset (nausea/vomiting/diarrhea), crystalluria/hematuria and acute kidney injury; seizures can occur especially with very high doses or renal impairment-manage with supportive care, ensure hydration/urine output, consider activated charcoal if early, and hemodialysis can enhance removal in severe cases.

Patient Counseling

Complete the full prescribed course and take doses at evenly spaced times. Take with or without food (food may reduce stomach upset) and swallow with water. Seek urgent care for allergy symptoms (rash/hives, swelling, wheeze/shortness of breath) and contact a clinician for severe/persistent watery diarrhea (possible C. difficile). Tell your prescriber if you have a history of penicillin/cephalosporin allergy or infectious mononucleosis (higher rash risk). Store below 25°C and keep out of reach of children; do not share antibiotics.

Pharmacology

Mechanism of Action

Inhibits the synthesis of bacterial cell walls by binding to penicillin-binding proteins, which prevents the final transpeptidation step of peptidoglycan synthesis, leading to cell lysis and death.

Duration of Effect

Approximately 6-8 hours for immediate‑release dosing (hence typical q8h or q12h regimens depending on indication/severity).

Half-Life

Approximately 1-1.5 hours (normal renal function).

Bioavailability

74-92%

Metabolism

Minimal metabolism; a small fraction is converted to inactive penicilloic acid.

Excretion

Primarily renal; about 60-80% excreted unchanged in urine (via glomerular filtration and tubular secretion).

Protein Binding

17-20%.

Product Information

Available Dosage Forms

Immediate‑release forms: capsules, tablets, chewable tablets, and powder for oral suspension; extended‑release tablets exist for amoxicillin (as a distinct ER product) in some markets.

Generic Availability

Yes

Spectrum

Broad-spectrum

Antibiotic Class

Penicillin

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