drugs classification used in std

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Drug Classification Used in Sexually Transmitted Diseases (STDs/STIs)

Below is a systematic classification of drugs used in STI treatment, organized by drug class and by disease. Based on Harrison's Principles of Internal Medicine 22E, Tintinalli's Emergency Medicine, and Rosen's Emergency Medicine.

1. Classification by Drug Class

A. Beta-Lactam Antibiotics (Cephalosporins & Penicillins)

Drug	Route	STI Indication
Ceftriaxone (3rd-gen cephalosporin)	IM (500 mg or 1g)	First-line for gonorrhea (N. gonorrhoeae); also used in chancroid, PID, gonococcal epididymitis
Benzathine Penicillin G	IM	First-line for all stages of syphilis (T. pallidum); 2.4 million units single dose for primary/secondary/early latent; 3 weekly doses for tertiary/late latent
Amoxicillin	PO	Alternative for chlamydia in pregnancy (500 mg TID x 7 days)

Key note: Fluoroquinolones are NO longer recommended for gonorrhea due to widespread resistance. Ceftriaxone is now the sole first-line agent. - Harrison's Internal Medicine 22E

B. Macrolides

Drug	Route/Dose	STI Indication
Azithromycin	1 g PO single dose	First-line for chlamydia (C. trachomatis); alternative for gonorrhea (with ceftriaxone), chancroid; safe in pregnancy
Azithromycin	2 g PO single dose	Alternative for gonorrhea
Erythromycin base	500 mg PO QID x 7 days	Alternative for chlamydia

C. Tetracyclines

Drug	Route/Dose	STI Indication
Doxycycline	100 mg PO BID x 7 days	First-line for chlamydia; used in PID (oral step-down); treatment of non-gonococcal urethritis (NGU); LGV (100 mg BID x 21 days); early syphilis (alternative if penicillin-allergic)
Tetracycline	500 mg PO QID x 28 days	Alternative for syphilis in penicillin allergy

Contraindicated in pregnancy - Rosen's Emergency Medicine

D. Nitroimidazoles

Drug	Route/Dose	STI Indication
Metronidazole	2 g PO single dose	First-line for trichomoniasis (T. vaginalis); also first-line for bacterial vaginosis (BV) (500 mg BID x 7 days or 0.75% vaginal gel x 5 days)
Tinidazole	2 g PO single dose	Alternative for trichomoniasis; alternative for BV

E. Fluoroquinolones

Drug	Route/Dose	STI Indication
Ciprofloxacin	500 mg PO single dose x 3 days	Used in chancroid (H. ducreyi); NOT used for gonorrhea anymore
Ofloxacin / Levofloxacin	PO	Alternative agents in PID (in penicillin-allergic patients)

Note: Fluoroquinolones are contraindicated in pregnancy. - Rosen's Emergency Medicine

F. Lincosamides

Drug	Route/Dose	STI Indication
Clindamycin	300 mg PO BID x 7 days	Alternative for bacterial vaginosis; component of PID regimens (900 mg IV)
Clindamycin vaginal cream 2%	5 g intravaginally x 7 days	First-line alternative for BV

G. Antivirals

Drug	Route/Dose	STI Indication
Acyclovir	400 mg PO TID x 7-10 days	First-line for primary genital herpes (HSV-1/2); suppressive therapy (400 mg BID)
Valacyclovir	1 g PO BID x 7-10 days	First-line for primary genital herpes; suppressive therapy (500 mg OD)
Famciclovir	250 mg PO TID x 7-10 days	Alternative for genital herpes

H. Antiretrovirals (HIV)

Drug Class	Examples	Use
NRTIs	Tenofovir, Emtricitabine (TDF/FTC)	HIV treatment (first-line backbone); PrEP (pre-exposure prophylaxis)
NNRTIs	Efavirenz, Rilpivirine	Combined with NRTI backbone
Integrase Inhibitors	Dolutegravir, Bictegravir	Preferred third agents in current HIV regimens
Protease Inhibitors	Darunavir	Used in ART
Entry Inhibitors	Cabotegravir (injectable)	Long-acting PrEP

I. Topical / Local Agents

Drug	Form	STI Indication
Podophyllin / Podophyllotoxin	Topical solution	Genital warts (HPV) - provider-applied
Imiquimod 5%	Topical cream	Genital warts (HPV) - patient-applied; immune modulator
Trichloroacetic acid (TCA)	Chemical	Wart ablation
Clotrimazole / Fluconazole	Topical/oral	Vulvovaginal candidiasis

2. Classification by Disease (Quick Reference)

STD	First-Line Drug(s)	Drug Class
Gonorrhea	Ceftriaxone 500 mg IM (single dose)	3rd-gen cephalosporin
Chlamydia	Doxycycline 100 mg BID x 7 days	Tetracycline
Syphilis	Benzathine Penicillin G 2.4 MU IM	Beta-lactam
Genital Herpes	Acyclovir / Valacyclovir	Antiviral (nucleoside analogue)
Trichomoniasis	Metronidazole 2 g PO single dose	Nitroimidazole
Bacterial Vaginosis	Metronidazole 500 mg BID x 7 days	Nitroimidazole
Chancroid	Azithromycin 1 g PO or Ceftriaxone 250 mg IM	Macrolide / Cephalosporin
LGV (C. trachomatis)	Doxycycline 100 mg BID x 21 days	Tetracycline
Donovanosis	Azithromycin 1 g/week OR Doxycycline	Macrolide / Tetracycline
HPV / Warts	Imiquimod (topical), Podophyllin	Immune modulator / Antimitotic
Candidiasis	Fluconazole 150 mg PO single dose	Azole antifungal
HIV	TDF/FTC + Dolutegravir (ART)	Antiretrovirals
PID	Ceftriaxone + Doxycycline ± Metronidazole	Combination

3. Special Considerations

Pregnancy:

Safe: Azithromycin, Amoxicillin, Cephalosporins, Acyclovir, Metronidazole (all trimesters), Penicillin
Avoid: Tetracyclines (doxycycline), Fluoroquinolones - Rosen's Emergency Medicine

Dual therapy (gonorrhea + chlamydia): Since co-infection is common (~30-40%), when treating gonorrhea with ceftriaxone, always cover chlamydia concurrently with doxycycline or azithromycin. - Tintinalli's Emergency Medicine

Penicillin allergy and syphilis: Doxycycline 100 mg BID x 14 days is the alternative; desensitization to penicillin is preferred in pregnancy since no proven equivalent exists.

Drug resistance:

Gonorrhea has acquired resistance to fluoroquinolones, older penicillins, and even azithromycin (monotherapy)
Metronidazole resistance in T. vaginalis is increasingly reported; use higher-dose tinidazole as alternative
Ceftriaxone remains the last reliable oral single-dose agent for gonorrhea

Sources: Harrison's Principles of Internal Medicine 22E (2025), Tintinalli's Emergency Medicine, Rosen's Emergency Medicine - Concepts and Clinical Practice

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