Bioavailability of IV vs Oral Doxycycline
Oral doxycycline has excellent bioavailability at approximately 90-100% in humans, making it essentially equivalent to IV administration for most clinical purposes.
Key Pharmacokinetic Differences
The FDA drug label for doxycycline indicates that the drug is "readily absorbed" and "well absorbed and tolerated" 1, 2. While the label does not explicitly state a numerical bioavailability percentage, the clinical pharmacology data demonstrates:
After IV Administration (100 mg over 1 hour):
- Peak concentration: 2.5 mcg/mL
- After 200 mg over 2 hours: 3.6 mcg/mL 1
After Oral Administration:
- The drug is "readily absorbed" with high lipid solubility 1, 2
- No clinically significant difference in clearance between oral and IV dosing was observed in pediatric patients 1
Clinical Implications
The CDC guidelines for both doxycycline PEP and tularemia treatment list identical dosing for IV and oral routes (e.g., "100 mg every 12 hrs IV or orally"), indicating therapeutic equivalence 3. This dosing interchangeability would not be recommended if there were clinically significant bioavailability differences.
The guidelines explicitly state: "Patients who are treated initially with an IV or IM antimicrobial can be transitioned to an oral regimen once they have defervesced and clinically improved" 3. This transition uses the same dose, further confirming equivalent bioavailability.
Important Caveats:
- Gastrointestinal absorption: While bioavailability is high, oral absorption can be affected by food (particularly dairy products and divalent cations like calcium, magnesium, iron)
- Esophageal irritation: The FDA label notes risks of "esophageal erosion and ulceration" with oral administration 4
- Clinical context matters: IV administration may be preferred in patients with severe illness, vomiting, or inability to take oral medications—not because of bioavailability differences, but due to practical considerations
Route Selection
Choose oral doxycycline as first-line for most infections given the excellent bioavailability, lower cost, and ease of administration. Reserve IV doxycycline for patients who cannot tolerate oral intake, have severe gastrointestinal dysfunction, or require immediate therapeutic levels in critical illness.