Antibiotics II, Antifungals and Antivirals

Including tuberculosis drugs

Pharmacology

Drug / class	Key points
Tetracyclines (doxycycline, minocycline)	Photosensitivity; pill esophagitis: full glass of water, stay upright 30 min, not at bedtime. Separate from antacids, calcium, iron. Avoid in pregnancy. Tooth staining in children <8, but short doxycycline courses (≤21 days) are safe at any age: it's the drug of choice for Rocky Mountain spotted fever. Updated
Macrolides (azithromycin, clarithromycin, erythromycin)	Atypical pneumonia, pertussis. QT prolongation, GI upset, liver injury. Clarithromycin and erythromycin interact with many drugs (statins, warfarin). They do not treat C. diff and can cause it.
Trimethoprim-sulfamethoxazole	UTI, MRSA skin infections, PJP prevention. Stop at first rash (Stevens-Johnson syndrome). ↑ K⁺, ↑ creatinine, crystals in urine (drink fluids), photosensitivity. ↑ INR with warfarin. Avoid near term and in infants <2 months.
Metronidazole	Anaerobes, bacterial vaginosis, trichomoniasis (treat partners), giardia. Metallic taste, dark urine, neuropathy with long use. Avoid alcohol during and for 3 days after (traditional advice). ↑ warfarin effect. No longer first-line for C. diff. Updated
Nitrofurantoin	Uncomplicated bladder infections only. Take with food; urine turns brown. Avoid if CrCl <30. Long-term: lung and liver reactions, neuropathy.
Clindamycin	Skin, anaerobes, dental. Highest C. diff risk. Full glass of water.
Linezolid	MRSA, VRE. Weak MAOI: serotonin syndrome with SSRIs; limit tyramine. Low platelets with long use.

C. diff treatment Updated

Fidaxomicin is preferred; oral vancomycin is an acceptable alternative (IDSA/SHEA 2021). Metronidazole only if neither is available. Stop the causing antibiotic if possible; contact precautions; soap and water; no antidiarrheal drugs unless ordered.

Antifungals

Nystatin: swish and swallow 4–5×/day; remove dentures first.
Fluconazole: QT, liver; ↑ warfarin and many drug levels.
Amphotericin B: fever, chills, rigors (premedicate as ordered), nephrotoxicity, low K⁺ and Mg²⁺. Lipid forms are less toxic.

Antivirals

Acyclovir / valacyclovir: herpes and shingles; hydrate (kidney crystals with IV); doesn't cure herpes.
Oseltamivir: influenza; most effective within 48 h of symptoms; also for prevention. (Baloxavir: single dose.)
HIV: treatment for everyone diagnosed, daily adherence; PrEP before exposure; PEP within 72 h after exposure.

Tuberculosis drugs (RIPE)

Rifampin	Orange-red urine, sweat and tears (stains soft contact lenses). Liver toxicity. Many interactions: weakens birth control pills, warfarin, HIV drugs: use backup contraception.
Isoniazid	Liver toxicity (report dark urine, jaundice, nausea; avoid alcohol; liver tests). Peripheral neuropathy: give pyridoxine (B6).
Pyrazinamide	Liver toxicity; ↑ uric acid (gout).
Ethambutol	Optic neuritis: check visual acuity and red-green color vision at baseline and regularly.

Ideally taken on an empty stomach, at the same time daily, often by directly observed therapy. Stopping early → drug resistance.