What are the standard oral and intravenous dosing regimens for commonly used antibiotics and antihistamines in a typical adult (≥18 years) with normal renal and hepatic function?

Standard Dosing for Common Antibiotics and Antihistamines in Adults

For adults with normal renal and hepatic function, use the following evidence-based dosing regimens for commonly prescribed antibiotics and antihistamines.

Common Antibiotics - Intravenous Dosing

Beta-Lactams and Beta-Lactamase Inhibitor Combinations

• Piperacillin-tazobactam: 3.375 g IV every 6 hours (increase to 3.375 g every 4 hours or 4.5 g every 6 hours for Pseudomonas aeruginosa infections) 1
• Ampicillin-sulbactam: 3 g IV every 6 hours 1
• Ticarcillin-clavulanate: 3.1 g IV every 6 hours 1

Carbapenems

• Meropenem: 1 g IV every 8 hours 1
• Imipenem-cilastatin: 500 mg IV every 6 hours or 1 g IV every 8 hours 1
• Ertapenem: 1 g IV every 24 hours 1
• Doripenem: 500 mg IV every 8 hours 1

Cephalosporins

• Cefazolin: 1–2 g IV every 8 hours 1
• Cefepime: 2 g IV every 8–12 hours 1
• Ceftriaxone: 1–2 g IV every 12–24 hours 1
• Ceftazidime: 2 g IV every 8 hours 1
• Cefotaxime: 1–2 g IV every 6–8 hours 1
• Cefoxitin: 2 g IV every 6 hours 1
• Cefuroxime: 1.5 g IV every 8 hours 1

Fluoroquinolones

• Ciprofloxacin: 400 mg IV every 12 hours 1
• Levofloxacin: 750 mg IV every 24 hours 1
• Moxifloxacin: 400 mg IV every 24 hours 1

Aminoglycosides (Once-Daily Dosing Preferred)

• Gentamicin or tobramycin: 5–7 mg/kg IV every 24 hours (based on adjusted body weight; requires serum concentration monitoring) 1
• Amikacin: 15–20 mg/kg IV every 24 hours (based on adjusted body weight; requires serum concentration monitoring) 1

Glycopeptides and Other Agents

• Vancomycin: 15–20 mg/kg IV every 8–12 hours (based on total body weight; requires serum concentration monitoring) 1
• Linezolid: 600 mg IV every 12 hours 1
• Aztreonam: 1–2 g IV every 6–8 hours 1
• Metronidazole: 500 mg IV every 8–12 hours or 1500 mg IV every 24 hours 1
• Tigecycline: 100 mg IV loading dose, then 50 mg IV every 12 hours 1

Anti-Staphylococcal Penicillins

• Nafcillin or oxacillin: 2 g IV every 4 hours (12 g per 24 hours in 6 divided doses for endocarditis) 1

Common Antibiotics - Oral Dosing

Fluoroquinolones

• Ciprofloxacin: 500 mg PO every 12 hours 1
• Levofloxacin: 250–500 mg PO every 24 hours 1

Trimethoprim-Sulfamethoxazole (Bactrim)

• Standard infections: 1 double-strength tablet (160 mg TMP/800 mg SMX) PO twice daily 2
• MRSA skin/soft tissue infections: 1–2 double-strength tablets PO twice daily for 7–10 days (use 2 tablets for severe infections) 2
• Prophylaxis: 1 double-strength tablet PO daily, or 1 double-strength tablet on 3 consecutive days per week 2

Amoxicillin-Clavulanate (Augmentin)

• Standard dose: 500 mg PO every 8 hours or 875 mg PO every 12 hours 3
• High-dose for resistant organisms: 2000 mg PO every 12 hours (using extended-release formulation) 3

Other Oral Agents

• Clarithromycin: 500 mg PO every 12 hours 1
• Fluconazole: 50–400 mg PO every 24 hours 1
• Acyclovir: 200–800 mg PO 5 times per day 1

Antihistamines - Standard Adult Dosing

While the provided evidence focuses primarily on antibiotics, standard antihistamine dosing for adults with normal organ function includes:

First-Generation Antihistamines

• Diphenhydramine: 25–50 mg PO/IV every 6 hours (maximum 400 mg/day)
• Hydroxyzine: 25–100 mg PO every 6 hours

Second-Generation Antihistamines

• Cetirizine: 10 mg PO once daily
• Loratadine: 10 mg PO once daily
• Fexofenadine: 60 mg PO twice daily or 180 mg PO once daily

Critical Dosing Considerations

Renal Impairment Adjustments

For creatinine clearance (CrCl) 15–30 mL/min, reduce most antibiotic doses by 50% or extend dosing intervals 2. Specific adjustments include:

• Trimethoprim-sulfamethoxazole: Reduce dose by 50% 2
• Levofloxacin: 500 mg loading dose, then 250 mg every 24 hours 1
• Fluconazole: Reduce dose by 50% 1

For CrCl <15 mL/min, further dose reduction or alternative agents are required 2.

Aminoglycoside and Vancomycin Monitoring

Serum drug concentration monitoring is mandatory for aminoglycosides and vancomycin to individualize dosing and prevent toxicity 1. Initial dosing should be based on adjusted body weight for aminoglycosides and total body weight for vancomycin 1.

Common Pitfalls to Avoid

• Underdosing beta-lactams in serious infections: Maximize doses when undrained abscesses or resistant organisms are suspected 1
• Failure to adjust for renal function: Not reducing doses when CrCl <30 mL/min significantly increases toxicity risk 2
• Inadequate hydration with high-dose sulfonamides: Ensure at least 1.5 L fluid intake daily to prevent crystalluria 2
• Using standard doses for resistant organisms: High-dose amoxicillin-clavulanate (90 mg/kg/day in children, proportional adult dosing) is required in areas with >10% penicillin-resistant S. pneumoniae 3