Overview
Doxycycline is a highly effective, broad-spectrum antibiotic belonging to the tetracycline class. It is widely prescribed for a diverse range of conditions, including respiratory tract infections (like atypical pneumonia), sexually transmitted infections (such as chlamydia), tick-borne diseases (like Lyme disease), and as a long-term treatment for moderate to severe acne and rosacea.

Mechanism of Action
Doxycycline acts as a bacteriostatic agent, meaning it stops bacteria from multiplying rather than killing them directly. It achieves this by entering the bacterial cell and reversibly binding to the 30S ribosomal subunit. This action prevents the addition of new amino acids to the growing peptide chain, effectively inhibiting the bacterial protein synthesis necessary for growth and reproduction.

Dosage and Administration
The dosage varies depending on the indication. For many acute infections, the typical adult dose is 100 mg every 12 hours on the first day, followed by 100 mg once daily. For acne, lower doses may be used long-term. Crucially, doxycycline should be taken with a full glass of water, and patients must remain upright (sitting or standing) for at least 30 minutes after taking it to prevent severe esophageal irritation and ulceration.

Side Effects
Common side effects include gastrointestinal upset, such as nausea, vomiting, and diarrhea. A notable side effect of doxycycline is pronounced photosensitivity, meaning patients can sunburn much more easily and severely; sun protection is essential. It can also cause vaginal yeast infections in women due to alterations in normal flora.

Contraindications and Precautions
It is contraindicated in individuals with a known hypersensitivity to any tetracycline antibiotic. Doxycycline strongly binds to calcium, iron, and antacids, which can drastically reduce its absorption; these should be taken several hours apart. It should be used with caution in patients with hepatic impairment.

Pregnancy and Lactation
Doxycycline is strictly contraindicated during the second and third trimesters of pregnancy, as well as in children under 8 years old. This is because the drug can permanently discolor developing teeth (turning them yellow-gray-brown) and temporarily inhibit bone growth. It is excreted in breast milk and is generally not recommended during lactation.