What happens when a 90-pound child ingests an adult dose of doxycycline (tetracycline antibiotic)?

Potential Adverse Effects of Adult Doxycycline Dose in a 90-Pound Child

A 90-pound child ingesting an adult dose of doxycycline could experience gastrointestinal distress, photosensitivity reactions, and other adverse effects, but is unlikely to experience life-threatening toxicity from a single dose. 1

Dosing Considerations

For children weighing less than 100 pounds (45 kg), the appropriate doxycycline dosage is:

• Initial dose: 2 mg/lb of body weight divided into 2 doses on first day
• Maintenance dose: 1 mg/lb of body weight daily (single dose or divided) 2

For a 90-pound child, this would equate to:

• Initial dose: 180 mg divided into two 90 mg doses
• Maintenance dose: 90 mg daily

In contrast, the adult dose is:

• Initial dose: 200 mg (100 mg every 12 hours)
• Maintenance dose: 100 mg daily 2

Potential Adverse Effects

Gastrointestinal Effects

• Nausea, vomiting, diarrhea
• Esophagitis and esophageal ulcerations (particularly if taken with insufficient water or immediately before lying down) 1, 3
• Anorexia, glossitis, dysphagia 2

Dermatological Effects

• Photosensitivity (exaggerated sunburn reaction) 2
• Maculopapular and erythematous rashes
• In rare cases: Stevens-Johnson syndrome, toxic epidermal necrolysis 2

Neurological Effects

• Dizziness, headache
• In rare cases: benign intracranial hypertension (pseudotumor cerebri) 2, 1

Other Potential Effects

• Hypersensitivity reactions (urticaria, angioneurotic edema)
• Dental effects (not from a single dose, but with prolonged use)
• Renal effects: rise in BUN (dose-related) 2

Risk Assessment

The risk of serious toxicity from a single adult dose in a 90-pound child is relatively low. Most adverse effects would be expected to be mild to moderate and self-limiting. The primary concerns would be:

1. Gastrointestinal distress - most common and immediate concern
2. Photosensitivity reactions - if exposed to sunlight
3. Esophageal irritation - if taken without adequate water

Management Recommendations

If a 90-pound child has ingested an adult dose of doxycycline:

1. Ensure adequate hydration - Give plenty of water to reduce risk of esophageal irritation 1
2. Monitor for symptoms - Watch for nausea, vomiting, abdominal pain
3. Avoid sun exposure - Protect from direct sunlight for 24-48 hours 2
4. Consider antacids - If GI symptoms develop (but separate administration by 2-3 hours from doxycycline) 2
5. Seek medical attention if severe symptoms develop (persistent vomiting, severe abdominal pain, difficulty swallowing)

Important Caveats

• While doxycycline has historically been contraindicated in children under 8 years due to concerns about dental staining, recent evidence suggests this risk may be overstated with short-term use 4, 5
• A single overdose is unlikely to cause dental effects, which are associated with prolonged use
• Doxycycline has a better safety profile than older tetracyclines 5
• The risk of serious adverse effects increases with multiple doses rather than a single exposure

If symptoms persist or worsen, medical evaluation is recommended to assess for rare but serious complications such as esophageal ulceration or hypersensitivity reactions.