Bioavailability of IV vs PO Doxycycline
In typical adults without malabsorption or severe GI disease, there is essentially no clinically significant difference in bioavailability between IV and oral doxycycline—oral absorption is rapid and virtually complete, making the two routes therapeutically equivalent. 1
Evidence for Equivalent Bioavailability
Oral doxycycline is rapidly and almost completely absorbed from the digestive tract, with approximately 75-95% bioavailability. 2, 1, 3
Serum concentrations following oral and intravenous administration are comparable, with no statistically significant differences observed in pharmacokinetic studies. 4, 1
Food does not significantly affect the absorption of doxycycline, unlike other tetracyclines. 2, 1
Clinical Implications from Guidelines
The Infectious Diseases Society of America's treatment recommendations reflect this equivalent bioavailability in practice:
For Lyme disease with neurologic manifestations, guidelines explicitly state that "doxycycline is well absorbed orally; thus, intravenous administration should only rarely be needed." 5
The recommendation is for 200-400 mg per day orally for 10-28 days, with IV reserved only for patients who cannot tolerate oral intake. 5
For severe infections like inhalational anthrax, guidelines recommend starting IV but transitioning to oral when clinically appropriate, with both routes counting toward the total 60-day treatment duration. 5, 6
When IV Route Is Preferred
Despite equivalent bioavailability, IV administration is indicated in specific clinical scenarios:
Severely ill hospitalized patients who are vomiting or obtunded and cannot reliably take oral medications. 7
Patients with active vomiting or ileus where oral absorption may be compromised. 5
Initial therapy in life-threatening infections (e.g., severe anthrax, meningitis) where immediate therapeutic levels are critical, though transition to oral should occur once the patient stabilizes. 5, 6
Important Caveats
In patients with malabsorption syndromes or severe gastrointestinal disease, oral bioavailability may be reduced or unpredictable, making IV administration preferable. 8
The question specifically excludes these populations, but in real practice, conditions like Crohn's disease, celiac disease, or short bowel syndrome can significantly alter doxycycline absorption. 8
Doxycycline has poor CNS penetration regardless of route, making it suboptimal for meningitis even when given IV. 5, 6