04 Antimicrobial Medications

How antimicrobial treatment is selected

Antimicrobial medications act against bacteria, viruses, fungi, or parasites. Their selection depends on the suspected organism, infection site, susceptibility, patient factors, and local guidance. Antimicrobials are not interchangeable: the right choice depends on diagnosis and current clinical guidance.

Antibacterial medications

Antibiotics treat bacterial infections, not viral illnesses such as colds or influenza. Unnecessary use can cause adverse effects and promote antimicrobial resistance. The listed drug–use associations are common examples, not universal first-line choices or treatment instructions. Familiar U.S. brand names are study aids; availability and branding can vary.

For confirmed group A strep throat, penicillin or amoxicillin are identified as options. For uncomplicated cystitis in eligible patients, nitrofurantoin, trimethoprim–sulfamethoxazole (TMP-SMX) in appropriate settings, or fosfomycin are identified as options.

Antibacterial classes and examples

Representative antibacterial classes, generic–brand pairs, and common associations include:

• Natural penicillins: Penicillin V (Veetids) and penicillin G (Bicillin L-A, for certain formulations) are associated with streptococcal infections; penicillin G is also used for syphilis.

• Aminopenicillins: Amoxicillin (Amoxil) is commonly used for confirmed strep throat. Amoxicillin–clavulanate (Augmentin) adds coverage for some beta-lactamase-producing bacteria, including in selected sinus and bite infections.

• Cephalosporins: Cephalexin (Keflex) is associated with skin infections and some urinary infections. Ceftriaxone (Rocephin) is associated with serious infections and selected sexually transmitted infections.

• Macrolides: Azithromycin (Zithromax) and clarithromycin (Biaxin) are associated with selected respiratory infections and atypical bacteria. Azithromycin is also used for some sexually transmitted infections, but resistance limits use for some infections.

• Tetracyclines: Doxycycline (Vibramycin) is associated with acne, tick-borne infections, and selected respiratory or skin infections.

• Fluoroquinolones: Ciprofloxacin (Cipro) and levofloxacin (Levaquin) are associated with selected gram-negative, urinary, or respiratory infections. They are often reserved for situations where safer alternatives are unsuitable.

• Sulfonamide combination: Trimethoprim–sulfamethoxazole (Bactrim, Septra) is associated with selected urinary and skin infections. It is also used to prevent or treat Pneumocystis pneumonia in at-risk patients.

• Nitrofurans: Nitrofurantoin (Macrobid) is associated with uncomplicated lower urinary tract infection; it is not a drug for kidney infection.

• Glycopeptides: Vancomycin (Vancocin) is associated with serious gram-positive infections, including MRSA. Oral vancomycin is used for C. difficile infection.

• Lincosamides: Clindamycin (Cleocin) is associated with selected skin, dental, and anaerobic infections. A notable association is C. difficile diarrhea.

• Oxazolidinones: Linezolid (Zyvox) is used for resistant gram-positive infections, including MRSA and VRE.

• Nitroimidazoles: Metronidazole (Flagyl) is associated with anaerobic bacterial infections and protozoal infections such as trichomoniasis.

Antiviral medications

Antiviral drugs act against viruses, with different groups associated with specific viral infections.

• Herpesvirus nucleoside analogues: Acyclovir (Zovirax), valacyclovir (Valtrex), and famciclovir (Famvir) are used for HSV, including genital herpes. They suppress outbreaks but do not eliminate latent virus. Acyclovir is also used for varicella-zoster infections.

• Influenza neuraminidase inhibitors: Oseltamivir (Tamiflu), zanamivir (Relenza), and peramivir (Rapivab) are used for influenza A and B. Oseltamivir is a commonly used oral option, and treatment is most effective when started early. Zanamivir is inhaled and is generally avoided in people with asthma or COPD.

• Influenza cap-dependent endonuclease inhibitor: Baloxavir (Xofluza) is used for uncomplicated influenza A or B and is given as a single oral dose in eligible patients.

• Antiretroviral combination: Bictegravir/emtricitabine/tenofovir alafenamide (Biktarvy) is used to treat HIV. Bictegravir is an integrase strand transfer inhibitor (INSTI); emtricitabine and tenofovir are nucleos(t)ide reverse transcriptase inhibitors (NRTIs). HIV is treated with combination antiretroviral therapy.

• SARS-CoV-2 antivirals: Nirmatrelvir/ritonavir (Paxlovid) and remdesivir (Veklury) are used to treat COVID-19 in eligible patients at increased risk of severe disease. Ritonavir has important drug interactions, and treatment timing and eligibility matter.

Antifungal medications

Antifungal drugs target fungal infections. Their classes and representative associations include:

• Azoles: Fluconazole (Diflucan) is associated with many Candida infections, including oral or esophageal candidiasis. Clotrimazole (Lotrimin) is used for topical or local yeast infections. Voriconazole (Vfend) is used for selected serious fungal infections. Azoles inhibit fungal ergosterol synthesis.

• Polyenes: Amphotericin B (AmBisome, formulation-dependent) is used for severe systemic fungal infections and can cause kidney toxicity. Nystatin (Mycostatin) is used for local candidiasis, such as oral thrush.

• Echinocandins: Caspofungin (Cancidas) and micafungin (Mycamine) are used intravenously for selected serious Candida infections. They inhibit fungal cell-wall synthesis.

• Squalene epoxidase inhibitors: Terbinafine (Lamisil) is associated with dermatophyte infections, especially nail or skin fungal infections.

• Other antifungals: Griseofulvin (Gris-PEG) is used for selected dermatophyte infections of the skin, hair, or nails.

Antiparasitic medications

Antiparasitic medications include drugs for protozoa, malaria, worms, and certain ectoparasites.

• Nitroimidazoles and antiprotozoals: Metronidazole (Flagyl) and tinidazole (Tindamax) are associated with trichomoniasis and giardiasis. Metronidazole also treats anaerobic bacterial infections.

• Antimalarial combinations: Artemether–lumefantrine (Coartem) and atovaquone–proguanil (Malarone) are used to treat malaria. Drug selection depends on parasite species, where the infection was acquired, and clinical severity.

• Antimalarial for relapse prevention: Primaquine prevents relapse in Plasmodium vivax or P. ovale malaria. G6PD testing is essential because of the risk of hemolysis.

• Benzimidazole anthelmintics: Albendazole (Albenza) and mebendazole (Emverm) are used for intestinal worms, including selected roundworm, hookworm, and pinworm infections.

• Anthelmintic and ectoparasiticide drugs: Ivermectin (Stromectol) treats certain parasitic worm infections and is used in some scabies regimens. Permethrin (Elimite) cream is a first-line topical scabies treatment.

• Other anthelmintic: Praziquantel (Biltricide) is used for selected fluke and tapeworm infections.

Names and review associations

A generic name identifies the active drug, while a brand name identifies a product. The same drug may have multiple brands or be sold generically. Representative associations to recall include penicillins, cephalosporins, macrolides, and tetracyclines among antibacterials; acyclovir or valacyclovir for herpes; oseltamivir for influenza; azoles and polyenes among antifungals; and metronidazole or tinidazole for selected protozoa.

• Antibiotic