Doxycycline Prophylactic Dose
For Lyme disease prevention after a high-risk tick bite, give a single 200 mg oral dose of doxycycline for adults and 4.4 mg/kg (maximum 200 mg) for children within 72 hours of tick removal 1.
Context-Specific Dosing
Lyme Disease Post-Tick Bite Prophylaxis
The most recent IDSA/AAN/ACR 2020 guidelines provide clear, evidence-based dosing:
- Adults: Single oral dose of 200 mg
- Children: Single oral dose of 4.4 mg/kg (up to maximum 200 mg)
- Timing: Must be administered within 72 hours of tick removal
- Indication: Only for high-risk bites meeting ALL three criteria:
- Identified Ixodes species tick
- Occurred in highly endemic area
- Tick attached ≥36 hours 1
Critical caveat: This single-dose regimen has 87% efficacy but is NOT recommended for routine use after all tick bites 2. The 2006 IDSA guidelines similarly recommended this dosing, with efficacy demonstrated in a landmark trial showing only 0.4% developed erythema migrans with prophylaxis versus 3.2% with placebo 3, 2.
STI Prevention (Doxy-PEP)
The 2024 CDC guidelines recommend doxycycline postexposure prophylaxis for bacterial STI prevention:
- Dose: 200 mg oral, single dose
- Timing: Within 72 hours after condomless oral, vaginal, or anal sex
- Maximum frequency: Not to exceed 200 mg every 24 hours
- Population: MSM and transgender women at risk for STIs 4
Important limitation: This is NOT effective in cisgender women, where a trial showed no significant reduction in STI incidence (relative risk 0.88,95% CI 0.60-1.29) 5. The efficacy is primarily against syphilis and chlamydia, with limited effect on gonorrhea due to resistance 4, 6.
Malaria Prophylaxis
For malaria prevention in endemic areas:
- Adults: 100 mg daily (NOT 200 mg)
- Children >8 years: 2 mg/kg daily (up to adult dose)
- Timing: Begin 1-2 days before travel, continue daily during exposure, and for 4 weeks after leaving endemic area 7, 8
Other Prophylactic Uses
Leptospirosis (flood exposure with laceration wounds): Single 200 mg dose showed 76.8% protective efficacy for infection and 86.3% for disease 9
Scrub typhus: 200 mg weekly during exposure periods in hyperendemic areas 10
Anthrax post-exposure: 100 mg twice daily for 60 days (children <45 kg: 2.2 mg/kg twice daily) 8
Key Clinical Pitfalls
Do NOT use 200 mg for malaria prophylaxis - the correct dose is 100 mg daily 8
Do NOT prescribe doxy-PEP for cisgender women - evidence shows no benefit 5
Do NOT give prophylaxis for low-risk tick bites - wait-and-watch approach is preferred when criteria aren't met 1
Contraindications for single-dose prophylaxis: Pregnancy, breastfeeding, children <8 years. Do NOT substitute amoxicillin for doxycycline in these populations for tick bite prophylaxis due to lack of effective short-course data 3
Timing is critical - efficacy drops significantly after 72 hours for both Lyme and STI prophylaxis 1, 4, 2
Common side effects with 200 mg single dose: Nausea (15.4%) and vomiting (5.8%), improved by taking with food 2