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Anti-STD Drugs - Pharmacology (MBBS Level)

Note: The KDT book (K.D. Tripathi's Essentials of Medical Pharmacology) is not in this library's collection, but the content below is drawn from Harrison's Principles of Internal Medicine 22nd Ed (2025), Goodman & Gilman's, Katzung's Basic & Clinical Pharmacology 16th Ed, and Creasy & Resnik's Maternal-Fetal Medicine, all of which cover the same pharmacology as KDT at MBBS level and align with the CDC 2021 STI Treatment Guidelines.

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Overview of Major STDs and Their Drug Treatment

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1. SYPHILIS - Drugs

Drug of Choice: Penicillin G

• T. pallidum has remained exquisitely sensitive to penicillin for over 75 years - no resistance has developed.
• Penicillin kills T. pallidum at very low concentrations, but requires prolonged exposure because the organism multiplies very slowly.

Dosage by Stage (CDC 2021 Guidelines)

Penicillin-Allergic Patients (non-pregnant)

• Doxycycline 100 mg PO twice daily x 14 days (primary/secondary/early latent)
• Tetracycline HCl 500 mg PO four times daily x 14 days
• For late latent: extend to 4 weeks
• For neurosyphilis: desensitize and treat with penicillin (no reliable alternative)

Special Notes

• Jarisch-Herxheimer reaction: Acute febrile reaction (fever, headache, myalgia) within 24 hours of first dose - due to massive spirochete lysis and cytokine release. NOT an allergic reaction. Managed with NSAIDs.
• Syphilis in pregnancy: Always use penicillin (desensitize if allergic); doxycycline/tetracycline are contraindicated.
• Source: Harrison's 22nd Ed; Campbell-Walsh Urology

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2. GONORRHEA - Drugs

Drug of Choice: Ceftriaxone (3rd-generation cephalosporin)

• Fluoroquinolones (ciprofloxacin) are no longer recommended due to widespread resistance.

Regimens (CDC 2021)

• Ceftriaxone 500 mg IM single dose + Doxycycline 100 mg PO x 7 days (for presumed co-infection with chlamydia)
• If weight >150 kg: Ceftriaxone 1 g IM

• Cefixime 800 mg PO single dose
• Gentamicin 240 mg IM + Azithromycin 2 g PO (single doses)

• Ceftriaxone 1 g IM or IV every 24 hours
• Alternatives: Cefotaxime 1 g IV q8h, or Ceftizoxime 1 g IV q8h
• Continue 24-48 hours after improvement, then switch to oral cefixime 400 mg twice daily
• Gonococcal meningitis/endocarditis: Ceftriaxone 1-2 g IV q12h

• Ceftriaxone + Azithromycin 1 g PO (instead of doxycycline; tetracyclines/quinolones are contraindicated)

• Erythromycin 0.5% ophthalmic ointment, or Tetracycline 1% ointment, or Silver nitrate 1% solution

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3. CHLAMYDIA - Drugs

Drug of Choice: Doxycycline (non-pregnant) / Azithromycin (pregnant)

• Doxycycline 100 mg PO twice daily x 7 days (preferred - superior efficacy for rectal chlamydia)

• Azithromycin 1 g PO single dose
• Amoxicillin 500 mg PO three times daily x 7 days

• Azithromycin 1 g PO single dose (DOC)
• Amoxicillin 500 mg PO TDS x 7 days
• Alternatives: Erythromycin base 500 mg QID x 7 days
• Erythromycin estolate is CONTRAINDICATED in pregnancy (hepatotoxicity)

• Doxycycline 100 mg PO twice daily x 21 days (longer course needed)

• Inhibit 30S ribosomal subunit; bacteriostatic
• Effective for chlamydial infections including STIs
• Also alternative agent for primary and secondary syphilis
• Source: Katzung's 16th Ed; Harrison's 22nd Ed

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4. GENITAL HERPES (HSV-1, HSV-2) - Drugs

Mechanism of Action: Nucleoside Analogues

Acyclovir

• Analogue of deoxyguanosine
• Phosphorylated to monophosphate by viral thymidine kinase (selective activation in infected cells)
• Cellular kinases convert it to active triphosphate form
• Inhibits viral DNA polymerase and is incorporated into viral DNA, causing chain termination
• Excreted by kidneys - dose reduction required in renal insufficiency
• IV form: Reversible renal insufficiency due to crystallization in renal tubules (ensure adequate hydration)
• CNS side effects more common at high levels

Valacyclovir

• Valine ester prodrug of acyclovir
• Much better oral bioavailability - plasma levels ~4x higher than oral acyclovir
• Rapidly converted to acyclovir in liver and intestine
• Additional indication: reduction of transmission of genital HSV

Famciclovir

• Diacetyl ester of penciclovir (converted to penciclovir in intestine and liver)
• Penciclovir = guanosine analogue, less potent than acyclovir but longer intracellular half-life
• Phosphorylated by HSV and VZV thymidine kinases
• Approved for: zoster, suppression of genital herpes, recurrent mucocutaneous herpes in HIV

Dosing Schedule (CDC Guidelines)

Resistance

• Mutations in viral thymidine kinase (most common) or viral DNA polymerase
• Resistant HSV/VZV treated with: Foscarnet or Cidofovir
• Rare in immunocompetent persons; more common in immunocompromised

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5. TRICHOMONIASIS

Drug of Choice: Metronidazole / Tinidazole

• Metronidazole 2 g PO single dose (preferred)
• Tinidazole 2 g PO single dose (alternative)
• Both sexual partners must be treated simultaneously
• Mechanism: Converted to reactive nitro radicals that damage protozoal DNA

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6. CHANCROID (Haemophilus ducreyi)

• Azithromycin 1 g PO single dose
• Ceftriaxone 250 mg IM single dose
• Ciprofloxacin 500 mg PO twice daily x 3 days

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7. GENITAL WARTS (Condylomata Acuminata - HPV)

Topical/Local Treatments

• Podophyllin (podophyllotoxin 0.5%): Anti-mitotic agent, arrests cell division in metaphase; applied by patient
• Imiquimod 5% cream: Immune response modifier (TLR-7 agonist); stimulates local interferon-alpha production
• Trichloroacetic acid (TCA): Chemical cautery
• Cryotherapy, laser, electrocautery (surgical/physical methods)
• HPV Vaccine (Gardasil 9): Primary prevention against HPV types 6, 11, 16, 18, 31, 33, 45, 52, 58

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8. HIV (STD Context)

• Post-exposure prophylaxis (PEP): Combination ART within 72 hours x 28 days
• Pre-exposure prophylaxis (PrEP): Tenofovir/Emtricitabine (TDF/FTC) daily

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Key Pharmacology Points for Exams (KDT Style Summary)

Contraindications to Remember

• Tetracyclines (doxycycline): Contraindicated in pregnancy and children <8 years
• Fluoroquinolones: No longer used for gonorrhea (resistance); avoid in pregnancy
• Erythromycin estolate: Contraindicated in pregnancy (hepatotoxicity)

Jarisch-Herxheimer Reaction

• Occurs within 6-8 hours of first dose of penicillin in syphilis
• Features: Fever, chills, headache, myalgia, rash exacerbation
• Mechanism: Massive release of endotoxin/cytokines from lysed spirochetes
• NOT an allergic reaction; managed with antipyretics (aspirin/paracetamol)
• More common in early syphilis (higher bacterial burden)

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