Clarithromycin exerts its antibacterial effect by inhibiting bacterial protein synthesis. It binds to the 50S subunit of the bacterial ribosome, preventing the translocation of peptides during translation. This action halts protein production, leading to bacteriostatic effects (inhibition of bacterial growth) or bactericidal effects (killing of bacteria), depending on the organism and drug concentration.

Clarithromycin is effective against a wide range of Gram-positive and Gram-negative bacteria, as well as some atypical pathogens. Its spectrum of activity includes:

• Gram-positive bacteria: Streptococcus pneumoniae, Streptococcus pyogenes, Staphylococcus aureus (methicillin-susceptible strains).
• Gram-negative bacteria: Haemophilus influenzae, Moraxella catarrhalis.
• Atypical pathogens: Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella pneumophila.
• Other organisms: Helicobacter pylori, Mycobacterium avium complex (MAC).

Clarithromycin is used to treat a variety of infections, including:

1. Respiratory Tract Infections:
   • Community-acquired pneumonia
   • Acute exacerbations of chronic bronchitis
   • Sinusitis
   • Pharyngitis and tonsillitis
2. Skin and Soft Tissue Infections:
   • Mild to moderate infections caused by susceptible organisms.
3. Helicobacter pylori Eradication:
   • Used in combination with other antibiotics (e.g., amoxicillin, metronidazole) and proton pump inhibitors (e.g., omeprazole) for peptic ulcer disease.
4. Mycobacterial Infections:
   • Prophylaxis and treatment of Mycobacterium avium complex (MAC) in immunocompromised patients (e.g., those with HIV/AIDS).
5. Other Infections:
   • Otitis media
   • Lyme disease (as an alternative to doxycycline)

Clarithromycin is available in oral formulations, including tablets, extended-release tablets, and oral suspensions. The dosage depends on the type and severity of the infection, patient age, and renal function.

Adults:

• Standard dose: 250–500 mg twice daily for 7–14 days, depending on the infection.
• Extended-release tablets: 500 mg once daily for 7–14 days.
• H. pylori eradication: 500 mg twice daily as part of combination therapy for 7–14 days.
• MAC prophylaxis: 500 mg twice daily.
• MAC treatment: 500 mg twice daily in combination with other antimycobacterial agents.

Children:

• Standard dose: 7.5 mg/kg twice daily (maximum 500 mg per dose) for 7–14 days.
• Extended-release tablets: Not recommended for children.

Renal Impairment:

• Dose adjustment is required for patients with severe renal impairment (creatinine clearance <30 mL/min). Reduce the dose by 50% or extend the dosing interval.

Clarithromycin is generally well-tolerated, but it can cause several adverse effects:

1. Gastrointestinal Effects:
   • Nausea, vomiting, diarrhea, abdominal pain, and dyspepsia are common.
   • Rarely, pseudomembranous colitis (caused by Clostridioides difficile) may occur.
2. Hepatotoxicity:
   • Elevated liver enzymes, hepatitis, and jaundice have been reported.
3. Cardiac Effects:
   • Prolonged QT interval and arrhythmias (e.g., torsades de pointes) may occur, particularly in patients with pre-existing cardiac conditions or those taking other QT-prolonging drugs.
4. Allergic Reactions:
   • Rash, pruritus, and, rarely, anaphylaxis.
5. Other Effects:
   • Headache, taste disturbances, and oral thrush.

1. Hepatic Impairment:
   • Use with caution in patients with liver disease. Monitor liver function tests regularly.
2. Renal Impairment:
   • Dose adjustment is required in severe renal impairment.
3. Cardiac Conditions:
   • Avoid in patients with a history of QT prolongation, arrhythmias, or heart failure.
4. Myasthenia Gravis:
   • Macrolides, including clarithromycin, may exacerbate muscle weakness.
5. Pregnancy and Lactation:
   • Use only if clearly needed. Clarithromycin crosses the placenta and is excreted in breast milk.
6. Drug Resistance:
   • Overuse or inappropriate use of clarithromycin can lead to the development of resistant bacteria.

Clarithromycin is a potent inhibitor of the cytochrome P450 enzyme system, particularly CYP3A4, leading to numerous drug interactions:

1. CYP3A4 Substrates:
   • Increased levels of drugs metabolized by CYP3A4, such as:
     • Statins (e.g., simvastatin, atorvastatin): Risk of myopathy and rhabdomyolysis.
     • Calcium channel blockers (e.g., verapamil, diltiazem): Risk of hypotension and bradycardia.
     • Immunosuppressants (e.g., cyclosporine, tacrolimus): Risk of nephrotoxicity.
2. QT-Prolonging Drugs:
   • Increased risk of arrhythmias when combined with other QT-prolonging drugs (e.g., fluoroquinolones, antipsychotics).
3. Warfarin:
   • Increased anticoagulant effect; monitor INR closely.
4. Digoxin:
   • Increased digoxin levels due to altered gut flora.
5. Ergot Alkaloids (e.g., ergotamine):
   • Risk of ergotism (severe vasoconstriction).
6. Antiretroviral Drugs:
   • Interactions with protease inhibitors (e.g., ritonavir) and non-nucleoside reverse transcriptase inhibitors (e.g., efavirenz).

Clarithromycin is available under various generic names, including:

• Clarithromycin
• Clarithromycin Extended-Release

It is marketed under several brand names, such as Biaxin, Klaricid, and Klaricid XL.