Nitrofurantoin
**Nitrofurantoin** is a synthetic nitrofuran antibacterial agent primarily used as a **urinary tract antiseptic**. * **Site-Specific Efficacy**: Because it is rapidly eliminated into the urine, it achieves high concentrations in the lower urinary tract but does *not* reach therapeutic concentrations in systemic circulation or tissues. Therefore, it is highly effective for **lower urinary tract infections (UTIs)** but is **ineffective** for systemic infections, pyelonephritis (kidney infections), or perinephric abscesses. * **Spectrum of Activity**: It is active against many gram-negative and gram-positive organisms, including *E. coli*, *Klebsiella*, *Staphylococcus*, and *Enterococcus*. It is notably ineffective against *Pseudomonas* spp. and most strains of *Proteus*. * **Clinical Pearl**: Due to increasing resistance to first-line antibiotics, nitrofurantoin is often reserved for multidrug-resistant lower UTIs, particularly those caused by resistant *E. coli* or *Enterococcus* species.
Mechanism: Nitrofurantoin can be bacteriostatic or bactericidal depending on the concentration and organism susceptibility. * **Mechanism**: Inside the bacterial cell, nitrofurantoin is reduced by bacterial **flavoproteins** to highly reactive intermediate compounds. * **Pathway**: Reactive intermediates โ attack bacterial **ribosomal proteins** and other macromolecules โ inhibit protein synthesis, aerobic energy metabolism (specifically **acetyl coenzyme A**), DNA synthesis, RNA synthesis, and cell wall synthesis. * **Environmental Factor**: It exhibits significantly greater antibacterial activity in **acidic environments**.
Dosing by species
- Susceptible bacterial urinary tract infections ยท 5 mg/kg ยท PO ยท q8h
- Recurrent UTI (Conventional dose) ยท 4 mg/kg ยท PO ยท q8h
- Recurrent UTI (Prophylactic dose) ยท 3-4 mg/kg ยท PO ยท q24h ยท Should be given at night after micturition and immediately before bedtime.
- Susceptible bacterial urinary tract infections ยท 4 mg/kg ยท PO ยท q6-8h
- Urinary tract infections (lower UTI only) ยท 4.4-5 mg/kg ยท PO ยท q8h ยท As directed by culture and sensitivity ยท For urinary tract infections only. Not for pyelonephritis.
- Susceptible urinary tract infections ยท 2.5-4.5 mg/kg ยท PO ยท q8h ยท Listed as three times daily.
- Susceptible urinary tract infections ยท 10 mg/kg ยท PO ยท q24h ยท Listed as daily.
- Prevention of re-infections with gram-negative organisms ยท 4 mg/kg ยท PO ยท q24h ยท Long-term ยท Give once a day immediately before bedtime after the dog has urinated. Only use after extensive search for underlying cause and after effective treatment of existing UTI.
- Recurrent UTI (Conventional dose) ยท 4 mg/kg ยท PO ยท q8h
Routes of administration
Contraindications
- Renal impairment (decreased efficacy and increased risk of systemic toxicity)
- Known hypersensitivity to nitrofurantoin
- Pregnant patients at term (risk of hemolytic anemia in neonates)
- Neonates
- Rats (high risk of neurotoxicity)
- Significant renal impairment (decreased excretion leads to toxic serum levels)
- Pregnancy (mutagenic properties)
- Pyelonephritis
- Systemic infections outside the urinary tract
- Food-producing animals
Adverse effects
- Gastrointestinal disturbances (vomiting, nausea, diarrhea)
- Hepatopathy (can be severe or chronic)
- Myasthenic-like effects (rare, reversible in dogs)
- Infertility in male dogs
- Peripheral neuropathy (rare)
- Hemolytic anemia (rare)
- Pneumonitis (rare)
- Emesis
- Diarrhoea
- Gastrointestinal bleeding
- Hepatotoxicity
- Peripheral neuritis (rare)
- Pulmonary complications (rare)
- Thrombocytopenia (at high doses)
- Anaemia (at high doses)
- Leucopenia (at high doses)
Drug interactions
- Fluoroquinolones (e.g., enrofloxacin, ciprofloxacin) ยท Nitrofurantoin may antagonize the antimicrobial activity of fluoroquinolones; concomitant use should be avoided.
- Food or Anticholinergic drugs ยท May increase the oral bioavailability and absorption of nitrofurantoin.
- Magnesium trisilicate containing antacids ยท May inhibit the oral absorption of nitrofurantoin.
- Probenecid ยท May inhibit the renal excretion of nitrofurantoin, potentially increasing systemic toxicity and reducing its effectiveness in the urinary tract.
- Antimuscarinic drugs ยท Delays gastric emptying time and increases absorption/bioavailability of nitrofurantoin ยท moderate
- Food ยท Delays gastric emptying time and increases absorption/bioavailability of nitrofurantoin ยท minor
- Fluoroquinolones ยท Antagonism of antibacterial efficacy ยท major
- Spironolactone ยท May increase the hyperkalaemic effects of spironolactone ยท moderate
- Urine alkalinizing agents ยท Significantly decreases the antibacterial activity of nitrofurantoin ยท major
Monitoring
- Clinical efficacy (resolution of UTI symptoms, negative urine cultures)
- Adverse effects (especially GI upset)
- Periodic liver function tests (especially with chronic/prophylactic therapy)
- Resolution of clinical signs of UTI
- Urine culture and sensitivity
- Renal function (BUN, Creatinine)
- Complete Blood Count (CBC) if used long-term or at high doses
- Liver enzymes
Overdose
Because the drug is rapidly absorbed and excreted, patients with normal renal function usually require little therapy for mild overdoses. * **Massive Overdose**: If ingestion was recent, empty the gut using standard protocols (emesis/gastric lavage). * **Monitoring**: Monitor closely for gastrointestinal distress, hepatotoxicity, and neurologic signs. Provide supportive care as needed.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.