What antibiotic is used to treat a spider bite?

Antibiotic Treatment for Spider Bites

For spider bites that become infected, amoxicillin-clavulanate is the recommended first-line oral antibiotic, while trimethoprim-sulfamethoxazole should be used if MRSA is suspected. 1, 2

Initial Assessment and Management

• Spider bites typically do not require antibiotics unless there are signs of secondary infection 3, 4
• Black widow spider bites primarily cause neuromuscular symptoms rather than local tissue damage, so antibiotics are not routinely indicated 3, 5
• Brown recluse spider bites may cause local necrosis but don't inherently require antibiotics unless secondary infection develops 4, 6
• Thoroughly irrigate the bite wound with clean water to reduce venom load and prevent infection 4

When Antibiotics Are Indicated

Signs of Secondary Infection

• Increasing erythema, warmth, swelling, purulence, or worsening pain beyond 24-48 hours 1, 2
• Systemic symptoms such as fever or lymphadenopathy 1, 2
• Failed initial wound care measures 2

Antibiotic Selection

First-line oral options:

• Amoxicillin-clavulanate (adults: 875/125 mg twice daily; children: 25 mg/kg/day of amoxicillin component in 2 divided doses) 1
• For suspected MRSA infection (increasingly common in spider bite infections):
  • Trimethoprim-sulfamethoxazole (1-2 double-strength tablets twice daily for adults) 1, 2
  • Doxycycline (100 mg twice daily) - not recommended for children under 8 years 1
  • Clindamycin (300-450 mg three times daily) 1

For severe infections requiring IV therapy:

• Ampicillin-sulbactam, piperacillin-tazobactam, or second-generation cephalosporins like cefoxitin 1
• For MRSA: vancomycin, linezolid, or daptomycin 1, 2

Special Considerations

• MRSA is increasingly common in spider bite infections - a 2003 study found 86.8% of cultured spider bite infections were MRSA 2
• Patients who fail initial outpatient therapy with penicillin-based oral antibiotics should be evaluated for MRSA infection 2
• Severe infections may require surgical debridement in addition to antibiotics 2
• Brown recluse spider bites with extensive necrosis may benefit from dapsone in addition to antibiotics, though this is controversial 6, 7

Common Pitfalls to Avoid

• Assuming all spider bites require antibiotics - most don't unless secondary infection develops 3, 4, 5
• Using first-generation cephalosporins or penicillinase-resistant penicillins alone when treating infected animal or spider bites 1
• Delaying aggressive treatment when MRSA is suspected - early surgical debridement and appropriate antibiotics are crucial 2
• Failing to obtain wound cultures in severe infections to guide antibiotic therapy 2