Antimicrobial Pharmacology MCQ

Dental antimicrobial pharmacology covers the agents a dentist actually prescribes for odontogenic infection, the alternatives for penicillin allergy, the high-yield interactions, and the drugs that show up in the medical history. Amoxicillin remains first-line for most dental infections; amoxicillin-clavulanate adds beta-lactamase coverage when needed. Clindamycin is the long-standing penicillin-allergy alternative but carries C. difficile risk. Metronidazole is the anaerobe agent and brings the disulfiram-like alcohol reaction and the classic warfarin INR rise. Doxycycline is avoided in pregnancy and in children under eight because of tooth staining and bone effects. Azole antifungals shoot up the INR on warfarin through CYP inhibition, and acyclovir is the standard antiviral for HSV reactivation.

Beta-Lactams: Penicillins and Cephalosporins

• Beta-lactams (penicillins, cephalosporins, carbapenems, monobactams) inhibit cell wall synthesis by binding penicillin-binding proteins (PBP), producing time-dependent killing.
• Amoxicillin is the dental workhorse: oral bioavailability, gram-positive and limited gram-negative coverage, used for most odontogenic infections at oral doses; amoxicillin-clavulanate adds clavulanate, a beta-lactamase inhibitor, for resistant organisms.
• True penicillin allergy is IgE-mediated and includes urticaria, angioedema, bronchospasm, and anaphylaxis; rashes from amoxicillin in mononucleosis are not true allergies.
• Cephalosporin cross-reactivity with penicillin is modest (~1-2 percent for first-generation; lower for later generations); cephalosporins are an option in non-severe penicillin allergy but are avoided after anaphylaxis or Stevens-Johnson syndrome.

Macrolides and Clindamycin

• Macrolides (azithromycin, clarithromycin, erythromycin) inhibit the 50S ribosomal subunit (bacteriostatic) and are used for respiratory and odontogenic infections; the high-yield concerns are QT prolongation and strong CYP3A4 inhibition (especially clarithromycin and erythromycin).
• Clindamycin also inhibits the 50S subunit and is a long-standing alternative for the penicillin-allergic patient and for anaerobic odontogenic infections; the major adverse effect is C. difficile colitis from disruption of normal gut flora.
• Azithromycin has the least CYP3A4 interaction of the macrolides and is sometimes preferred when the drug interaction profile matters.
• All three drugs (azithromycin, clarithromycin, erythromycin) can prolong QT and raise the risk of torsades de pointes when combined with other QT-prolonging drugs.

Tetracyclines, Metronidazole, Fluoroquinolones

• Tetracyclines (doxycycline, minocycline, tetracycline) inhibit the 30S ribosomal subunit; doxycycline is used for periodontal disease and for some respiratory infections. They are AVOIDED in pregnancy and in children under eight years old because they chelate calcium and stain developing teeth and impair bone growth.
• Tetracyclines also bind divalent cations (calcium, magnesium, iron, aluminum), so they must not be taken with dairy, antacids, or iron supplements that block absorption; photosensitivity is another class effect.
• Metronidazole damages bacterial DNA in anaerobes; in dentistry it is sometimes added to amoxicillin for serious anaerobic odontogenic infections. The two high-yield interactions are a disulfiram-like reaction with alcohol (flushing, nausea, palpitations) and a strong potentiation of warfarin with INR elevation.
• Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) inhibit DNA gyrase and topoisomerase IV. They are not routine dental drugs and carry tendon rupture, QT prolongation, peripheral neuropathy, and aortic dissection warnings; they are also avoided in pregnancy and in young children.

Antifungals: Azoles, Nystatin, Amphotericin

• Azole antifungals (fluconazole, itraconazole, ketoconazole, voriconazole) inhibit fungal ergosterol synthesis via the fungal CYP enzyme; the high-yield clinical issue is that they also inhibit human CYP3A4 (and CYP2C9), raising the plasma levels of many drugs including warfarin (a sharp INR rise) and statins (rhabdomyolysis risk).
• Nystatin is a polyene antifungal used topically as an oral suspension or pastille for oral candidiasis (it is not absorbed systemically).
• Amphotericin B is the broad-spectrum systemic polyene reserved for serious invasive fungal infections; it is nephrotoxic and infusion-reaction prone.
• Oral candidiasis in a denture wearer or in an inhaled-corticosteroid user is treated with topical nystatin or oral fluconazole (a few days for uncomplicated thrush), with attention to the underlying cause and to drug interactions.

Antivirals and Dental Prescribing

• Acyclovir is a nucleoside analog activated by viral thymidine kinase and incorporated into viral DNA, blocking replication. It is the first-line antiviral for HSV reactivation (herpes labialis, intraoral primary herpes); valacyclovir is the prodrug with better oral bioavailability.
• Early initiation of antiviral therapy (within 72 hours of prodrome or within 1 hour of lesion appearance for episodic herpes labialis) shortens the course; topical antivirals are weaker.
• Antibiotic dental prescribing for odontogenic infection: source control (incision and drainage, extraction, root canal) is the cornerstone; antibiotics are an adjunct for systemic signs, spreading infection, or in the immunocompromised. Amoxicillin remains first-line.
• Infective endocarditis prophylaxis (the drug-pharmacology detail): amoxicillin 2 g orally 30 to 60 minutes before the dental procedure in adults (50 mg/kg in children) for the specified high-risk cardiac conditions and procedures that manipulate gingiva or the periapex or perforate mucosa. Clindamycin 600 mg is NO LONGER the recommended penicillin-allergy alternative; current guidance is azithromycin 500 mg or a cephalexin (if not severely PCN-allergic) instead, with doxycycline as an option.