What is the typical dosing regimen for amoxicillin (amoxicillin) in an adult patient?

Amoxicillin Dosing for Adults

For most common bacterial infections in adults, amoxicillin should be dosed at 500 mg three times daily or 875-1000 mg twice daily for 7-10 days, with higher doses (2000 mg twice daily or 1000 mg three times daily) reserved for resistant organisms or severe infections. 1

Standard Adult Dosing Regimens

The FDA-approved dosing range for adults is 750 to 1750 mg/day divided every 8 to 12 hours 1. This translates to the following practical regimens:

For mild to moderate infections:

• 500 mg three times daily (every 8 hours) - This is the most common regimen for respiratory tract infections, skin infections, and genitourinary infections 2, 1
• 875-1000 mg twice daily (every 12 hours) - Alternative regimen with similar efficacy and improved compliance 3

For severe infections or resistant organisms:

• 1000 mg three times daily - Used for severe respiratory infections or when resistant Streptococcus pneumoniae is suspected 2, 4
• 2000 mg twice daily - Reserved for high-risk patients with recent antibiotic use, age >65, or areas with >10% penicillin-resistant S. pneumoniae 4

Specific Clinical Indications

Streptococcal pharyngitis:

• 500 mg twice daily for 10 days 2
• Alternative: 250 mg four times daily for 10 days 2

Acute bacterial sinusitis:

• Standard dose: 500 mg three times daily for 5-7 days 2, 4
• High-dose for risk factors: 1000 mg three times daily for 5-7 days 2, 4
• Risk factors requiring high-dose include: recent antibiotic use within 4-6 weeks, moderate-to-severe disease, age >65, immunocompromised status, or geographic areas with high resistance rates 4

Community-acquired pneumonia:

• Mild-to-moderate: 500 mg three times daily for 7-10 days 2
• Severe or resistant pathogens: 1000 mg three times daily 2

Lyme disease (erythema migrans):

• 500 mg three times daily for 14-21 days 2

Helicobacter pylori eradication:

• Triple therapy: 1000 mg twice daily with clarithromycin 500 mg twice daily and a proton pump inhibitor for 14 days 1
• Dual therapy: 1000 mg three times daily with a proton pump inhibitor for 14 days 1
• Critical point: Amoxicillin should be dosed at least three times daily for H. pylori eradication, as more frequent dosing (tid or qid) achieves significantly higher eradication rates (90-95%) compared to twice-daily dosing (78-80%) 5

Duration of Therapy

The trend toward shorter courses is supported by high-quality evidence:

• Acute bacterial sinusitis: 5-7 days is as effective as 10 days 2, 4, 6
• Community-acquired pneumonia: 5 days for patients showing early clinical improvement is non-inferior to 10 days 2
• Streptococcal pharyngitis: 10 days remains standard 2

When to Use Amoxicillin-Clavulanate Instead

Switch to amoxicillin-clavulanate (not plain amoxicillin) when:

• β-lactamase-producing organisms suspected (H. influenzae, M. catarrhalis, S. aureus) 4, 7
• Treatment failure with amoxicillin alone 4
• Concurrent purulent conjunctivitis (suggests M. catarrhalis) 7
• Animal or human bite wounds 4

Renal Dosing Adjustments

For patients with severe renal impairment (GFR <30 mL/min), reduce the dose or extend the dosing interval 1. Specific adjustments should be based on creatinine clearance, though the FDA label does not provide exact regimens for all levels of renal dysfunction 1.

Common Pitfalls to Avoid

Underdosing is a critical error: Using 250 mg doses in adults provides inadequate serum concentrations and promotes antimicrobial resistance 2, 1. The minimum effective adult dose is 500 mg per administration 2, 1.

Twice-daily dosing for H. pylori: This achieves significantly lower eradication rates (77-80%) compared to three or four times daily dosing (90-95%) because amoxicillin's bactericidal activity depends on time above MIC 5.

Ignoring resistance patterns: In areas with >10% penicillin-resistant S. pneumoniae, standard amoxicillin doses are inadequate and high-dose regimens (1000 mg three times daily) or amoxicillin-clavulanate should be used 4, 6.

Unnecessary prolonged courses: For acute bacterial sinusitis and community-acquired pneumonia, 5-7 day courses are equally effective as 10-day courses in most patients, reducing adverse effects and resistance 2, 6.