Usage
Doxycycline is prescribed for a wide range of bacterial infections, including respiratory tract infections (e.g., pneumonia, bronchitis), urinary tract infections, skin infections (e.g., acne, cellulitis), sexually transmitted infections (e.g., chlamydia, gonorrhea, syphilis), and certain tick-borne illnesses (e.g., Lyme disease, Rocky Mountain spotted fever). It is also used for malaria prophylaxis and in the treatment of rosacea. Its pharmacological classification is tetracycline antibiotic. Doxycycline exerts its bacteriostatic effect by inhibiting bacterial protein synthesis by binding to the 30S ribosomal subunit, preventing the binding of aminoacyl-tRNA to the messenger RNA-ribosome complex.
Alternate Names
Vibramycin, Doryx, Oracea, Atridox, Acticlate, Morgidox, Periostat
How It Works
Pharmacodynamics: Doxycycline inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit. This prevents the addition of amino acids to the growing peptide chain, thereby inhibiting bacterial growth and replication.
Pharmacokinetics:
- Absorption: Doxycycline is well-absorbed orally, with bioavailability exceeding 90%. Absorption is slightly reduced by food, milk, and antacids containing aluminum, calcium, or magnesium.
- Distribution: Doxycycline is widely distributed throughout the body, achieving therapeutic concentrations in various tissues and fluids, including lungs, skin, and genitourinary tract. It exhibits high protein binding (approximately 90%).
- Metabolism: Doxycycline is primarily excreted unchanged in the urine and feces. A small portion is metabolized in the liver.
- Elimination: Doxycycline is eliminated primarily through renal and fecal excretion. The elimination half-life is approximately 18-22 hours, allowing for once or twice-daily dosing.
Mode of Action: Doxycycline’s mode of action involves binding to the 30S ribosomal subunit, specifically the 16S rRNA. This binding blocks the attachment of aminoacyl-tRNA to the A site of the ribosome, halting the elongation of the polypeptide chain. It does not directly kill bacteria (bactericidal) but inhibits their growth (bacteriostatic).
Elimination Pathways: Doxycycline is excreted primarily via the renal route (approximately 40% in 72 hours in individuals with normal renal function) and also in the feces.
Dosage
Standard Dosage
Adults:
- Most infections: 100 mg every 12 hours on the first day, followed by 100 mg once daily or 50 mg every 12 hours.
- More severe infections: 100 mg every 12 hours.
Children (over 8 years and at least 45 kg): Same as adult dosing.
Children (over 8 years and under 45 kg): 4.4 mg/kg/day in two divided doses on the first day, followed by 2.2 mg/kg/day in one or two divided doses. Maximum single dose: 100 mg.
Special Cases:
- Elderly Patients: No specific dosage adjustments are typically required, but caution is advised in patients with renal or hepatic impairment.
- Patients with Renal Impairment: No dosage adjustment is necessary for doxycycline in patients with renal impairment, as it is primarily excreted in the feces. However, monitor for drug accumulation in severe renal failure.
- Patients with Hepatic Dysfunction: Administer with caution in patients with hepatic impairment.
Clinical Use Cases
Doxycycline’s standard dosage guidelines apply to a broad range of infections and do not typically require modification in the settings of intubation, surgical procedures, mechanical ventilation, ICU use, or emergency situations. However, dose adjustments might be necessary in certain infections (e.g., severe or life-threatening infections) which should be done at the discretion of the physician. Prophylaxis prior to surgery often involve a single preoperative dose (e.g., 200 mg orally one hour before the procedure).
Dosage Adjustments
No specific dosage adjustments are required for renal impairment. Dose adjustment might be necessary in patients with hepatic impairment.
Side Effects
Common Side Effects
Nausea, vomiting, diarrhea, abdominal pain, photosensitivity (increased sensitivity to sunlight), dizziness, headache.
Rare but Serious Side Effects
Severe allergic reactions (anaphylaxis), Clostridium difficile-associated diarrhea, intracranial hypertension (pseudotumor cerebri), liver injury, skin reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis).
Long-Term Effects
Tooth discoloration (especially in children under 8 years of age), esophageal ulceration.
Contraindications
Hypersensitivity to tetracyclines, pregnancy (especially second and third trimesters), breastfeeding, children under 8 years of age (except in life-threatening conditions).
Drug Interactions
Antacids containing aluminum, calcium, or magnesium, iron supplements, bismuth subsalicylate, warfarin, barbiturates, phenytoin, carbamazepine.
Pregnancy and Breastfeeding
Doxycycline is contraindicated during pregnancy and breastfeeding due to the risk of fetal skeletal development issues and tooth discoloration in the infant.
Drug Profile Summary
- Mechanism of Action: Inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit.
- Side Effects: Nausea, vomiting, diarrhea, photosensitivity, dizziness, headache (common); C. difficile infection, intracranial hypertension, liver injury, severe skin reactions (rare but serious).
- Contraindications: Hypersensitivity to tetracyclines, pregnancy, breastfeeding, children under 8 years.
- Drug Interactions: Antacids, iron supplements, warfarin, anticonvulsants.
- Pregnancy & Breastfeeding: Contraindicated.
- Dosage: Varies based on indication and patient factors; refer to detailed dosage guidelines above.
- Monitoring Parameters: Clinical response to infection, signs of adverse effects.
Popular Combinations
Doxycycline is often used as monotherapy. Combination therapy may be indicated in specific infections.
Precautions
Avoid prolonged sun exposure, maintain adequate hydration, monitor for signs of adverse effects, obtain a thorough medical history including allergies and pre-existing conditions before initiating therapy.
FAQs (Frequently Asked Questions)
Q1: What is the recommended dosage for Doxycycline?
A: Refer to detailed dosage guidelines above, as dosing varies based on age, weight, indication, and other factors.
Q2: Can doxycycline be used in pregnant women?
A: No, doxycycline is contraindicated during pregnancy due to potential adverse effects on fetal skeletal development and teeth.
Q3: What are the common side effects of doxycycline?
A: Common side effects include gastrointestinal disturbances (nausea, vomiting, diarrhea), photosensitivity, dizziness, and headache.
Q4: How does doxycycline interact with antacids?
A: Antacids containing aluminum, calcium, or magnesium can reduce the absorption of doxycycline. Administer doxycycline at least 2 hours before or 4 hours after antacids.
Q5: Is dosage adjustment needed for renal impairment?
A: No, dosage adjustment is generally not necessary for renal impairment because doxycycline is primarily eliminated in the feces. However, monitor for drug accumulation in patients with severe renal failure.
Q6: What is the duration of doxycycline treatment for most infections?
A: The duration of treatment varies based on the infection being treated. Most infections require 7-14 days of therapy, but some may require longer durations.
Q7: Can doxycycline be used for malaria prophylaxis?
A: Yes, doxycycline can be used for malaria prophylaxis. The recommended dose is 100 mg once daily, starting 1-2 days before travel and continuing for 4 weeks after returning.
Q8: Can doxycycline cause tooth discoloration?
A: Yes, doxycycline can cause permanent tooth discoloration in children under 8 years of age. It is therefore contraindicated in this age group except for life-threatening conditions when no other alternatives are available. It can also cause tooth discoloration in pregnant women during the second and third trimesters, potentially affecting the deciduous teeth of the developing fetus.
Q9: What should patients be advised regarding sun exposure while taking doxycycline?
A: Patients taking doxycycline should be advised to avoid prolonged sun exposure and use sunscreen and protective clothing due to the risk of photosensitivity. They should also be instructed to discontinue doxycycline and seek medical advice if they experience any signs of photosensitivity, such as sunburn or skin rash.
