The blue capsule imprinted with CY on the cap and 186 on the body is an oral prescription broad-spectrum antibiotic containing Doxycycline Hyclate (100 mg).
Pill Identification
Attribute – Specification
- Active Ingredient – Doxycycline Hyclate, USP (equivalent to 100 mg doxycycline)
- Imprint – CY on the cap, 186 on the body
- Color & Shape – Light blue / blue, two-piece hard gelatin capsule
- Primary Manufacturers / Distributors – Chartwell RX / Bi-Coastal Pharma International
- Drug Class – Tetracycline-class antibacterial agent
- Prescription Status – Rx-only (Non-controlled)
Mechanism of Action
Doxycycline is a broad-spectrum, bacteriostatic antibiotic that halts bacterial growth by inhibiting protein synthesis:
- Cellular Entry: Doxycycline passively diffuses through outer membrane porin channels in Gram-negative bacteria and is actively transported across the inner cytoplasmic membrane in both Gram-positive and Gram-negative organisms.
- Ribosomal Subunit Binding: Inside the bacterial cell, it reversibly binds to the 30S ribosomal subunit (specifically near the A-site).
- Blockade of Translation: This binding sterically blocks aminoacyl-tRNA from accessing the ribosomal acceptor site, preventing the addition of new amino acids to the elongating peptide chain and arresting bacterial protein synthesis.
- Anti-inflammatory / Matrix Metalloproteinase (MMP) Inhibition: Independent of its antimicrobial properties, doxycycline inhibits collagenase and matrix metalloproteinases , providing anti-inflammatory benefits in dermatologic conditions such as severe acne and rosacea.
Indications and Clinical Uses
- Respiratory Tract Infections: Community-acquired pneumonia (CAP), atypical pneumonia (Mycoplasma pneumoniae, Chlamydia pneumoniae), and acute exacerbations of chronic bronchitis.
- Tick-Borne Rickettsial & Bacterial Illnesses: First-line treatment for Lyme disease (Borrelia burgdorferi), Rocky Mountain spotted fever (Rickettsia rickettsii), anaplasmosis, and ehrlichiosis.
- Sexually Transmitted Infections (STIs): Non-gonococcal urethritis, cervicitis, and lymphogranuloma venereum caused by Chlamydia trachomatis, as well as syphilis (in penicillin-allergic patients).
- Dermatologic Conditions: Moderate to severe inflammatory acne vulgaris and rosacea.
- Vector-Borne Prophylaxis & Anthrax: Malaria prophylaxis (Plasmodium falciparum) in chloroquine-resistant areas and post-exposure prophylaxis for inhalational Bacillus anthracis.
Dosage and Administration
- Doxycycline must always be taken with a full glass of water and the patient must remain upright for at least 30 minutes to prevent pill-induced esophageal ulceration.
- Standard Adult Dosing (General Infections):
- Initial Day: 100 mg orally every 12 hours (200 mg/day).
- Maintenance: 100 mg once daily or 100 mg every 12 hours for severe infections.
- Lyme Disease (Early Localized): 100 mg orally twice daily for 10 to 14 days.
- Malaria Prophylaxis: 100 mg orally once daily, starting 1 to 2 days before travel to an endemic area, continuing daily throughout the stay, and for 4 weeks after leaving.
- Food & Divalent Cations: While doxycycline is better absorbed with food than older tetracyclines (and taking it with a light meal reduces nausea), it should not be co-administered with calcium-rich dairy products or polyvalent cation supplements (iron, magnesium, aluminum, zinc), which form unabsorbable chelates.
Side Effects
Common Adverse Reactions:
- Gastrointestinal: Nausea, vomiting, epigastric discomfort, anorexia, diarrhea
- Dermatologic: Severe photosensitivity (exaggerated sunburn reactions with erythema and blistering upon minimal UV exposure)
- Mucocutaneous: Oral or vaginal candidiasis (yeast overgrowth)
Serious Adverse Reactions:
- Esophageal Irritation & Ulceration: Severe retrosternal pain and dysphagia caused by capsules lodging in the esophagus.
- Intracranial Hypertension (Pseudotumor Cerebri): Elevated intracranial pressure presenting with severe headache, blurred vision, diplopia, and papilledema (most common in women of childbearing age who are overweight).
- Clostridioides difficile-Associated Diarrhea (CDAD): Severe pseudomembranous colitis resulting from disruption of normal intestinal flora.
- Severe Cutaneous Adverse Reactions (SCAR): Stevens-Johnson syndrome (SJS), Toxic Epidermal Necrolysis (TEN), and Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS).
Key Drug Interactions
- Polyvalent Cations / Antacids: Co-administration with antacids containing aluminum, calcium, or magnesium, as well as oral iron preparations, bismuth subsalicylate, or zinc supplements, impairs doxycycline absorption via chelation. Doses should be separated by at least 2 to 3 hours.
- Anticonvulsants / Hepatic Inducers: CYP-inducing medications (e.g., carbamazepine, phenytoin, phenobarbital) significantly shorten the half-life of doxycycline, often requiring dosage adjustments.
- Oral Anticoagulants (Warfarin): Doxycycline depresses plasma prothrombin activity, potentiating the anticoagulant effect of warfarin and increasing bleeding risk.
- Penicillins & Bactericidal Agents: Bacteriostatic agents like doxycycline can antagonize the bactericidal action of cell-wall-active penicillins and cephalosporins.
- Oral Retinoids (Isotretinoin, Acitretin): Concurrent use is strictly contraindicated due to a synergistically heightened risk of benign intracranial hypertension.
Warnings and Precautions
- Tooth Enamel Hypoplasia & Skeletal Growth Retardation: Use during tooth development (the second half of pregnancy, infancy, and childhood up to 8 years of age) may cause permanent yellow-gray-brown discoloration of the teeth and reversible inhibition of bone growth. (Exception: Short courses are recommended regardless of age for life-threatening conditions like Rocky Mountain spotted fever and early Lyme disease).
- Esophageal Ulceration Warning: Always swallow capsules whole with plenty of fluids while sitting or standing; never take immediately before reclining or going to bed.
- Photosensitivity Warning: Patients should avoid excessive exposure to direct sunlight and UV tanning equipment; use broad-spectrum sunscreens and protective clothing during therapy.
- Expired Medication Toxicity (Fanconi-like Syndrome): Ingesting degraded or expired tetracyclines can cause severe renal tubular acidosis (Fanconi syndrome); discard all unused medication past the expiration date.