Prophylactic Dosage for Lyme Disease After Tick Bite
For adults, give a single dose of 200 mg doxycycline orally; for children ≥8 years, give a single dose of 4 mg/kg (maximum 200 mg) orally, but ONLY if strict criteria are met. 1, 2, 3
Strict Criteria That Must ALL Be Met
Prophylaxis should only be administered when every single one of these conditions is satisfied 1, 3:
- Tick identification: Must be an adult or nymphal Ixodes scapularis tick (deer tick) 1, 3
- Attachment duration: Tick was attached for ≥36 hours (can estimate by degree of engorgement) 1, 2, 3
- Timing: Prophylaxis must be given within 72 hours of tick removal 1, 2, 3
- Geographic endemicity: Bite occurred in a highly endemic area where ≥20% of ticks are infected with Borrelia burgdorferi (parts of New England, mid-Atlantic states, Minnesota, Wisconsin) 1, 3
- No contraindications: Patient can safely receive doxycycline 1, 3
If even one criterion is not met, do NOT give prophylaxis—use observation instead. 1
Exact Dosing Regimen
- Adults: Single dose of 200 mg doxycycline orally 1, 3
- Children ≥8 years: Single dose of 4.4 mg/kg orally (maximum 200 mg) 1, 3
This single-dose regimen has been validated in clinical trials showing 67% relative risk reduction for Lyme disease development, with a number-needed-to-treat of 51. 4, 5
Absolute Contraindications
- Pregnant women: Doxycycline is relatively contraindicated 1, 2, 3
- Children <8 years old: Doxycycline is relatively contraindicated 1, 2, 3
Critical pitfall: Do NOT substitute amoxicillin or other antibiotics for prophylaxis in patients with doxycycline contraindications—there is no evidence supporting effective short-course prophylaxis with alternative agents. 1 For these patients, use observation only.
When Prophylaxis Is NOT Recommended
- Ixodes pacificus (Western black-legged tick) bites generally do not require prophylaxis due to low infection rates (<20%) in most endemic regions 1
- Routine prophylaxis for all tick bites is not recommended 1, 3
- If the tick cannot be identified with certainty as high-risk, use a wait-and-watch approach 3
Post-Prophylaxis Monitoring
Even after prophylaxis, patients must be monitored for 30 days for 1, 2, 3:
- Erythema migrans: Expanding "bullseye" rash at the bite site or elsewhere
- Systemic symptoms: Flu-like symptoms, headache, muscle pain, joint pain, swollen lymph nodes
The single prophylactic dose does not guarantee prevention—patients should seek immediate medical attention if an expanding rash or systemic symptoms develop. 1
Critical Timing Window
The 72-hour window from tick removal is crucial for efficacy. 1, 2 After 72 hours, prophylaxis is no longer indicated, and observation becomes the appropriate strategy.
Common Pitfalls to Avoid
- Do not assume prior Lyme disease provides immunity: Patients with previous Lyme disease can be reinfected and require the same prophylaxis dosing if criteria are met 1
- Do not delay tick removal: Prompt removal reduces transmission risk, as attachment for <36 hours significantly decreases infection probability 2, 3
- Do not prescribe prophylaxis for non-Ixodes ticks: Other tick species do not transmit Lyme disease 1, 3