Side effects of sulfa drugs.
For sulfa antibiotics, the most common side effects are gastrointestinal upset, rash, and photosensitivity. Less common but clinically important: hyperkalemia and a benign rise in serum creatinine on TMP-SMX, hepatic injury, blood dyscrasias, and rare severe cutaneous reactions. The non-antibiotic sulfonamides have their own profiles, mostly tied to their pharmacology rather than to allergy.
- Most common
- Nausea, abdominal discomfort, maculopapular rash, photosensitivity.
- Less common, important
- Hyperkalemia (TMP-SMX), elevated creatinine without true GFR fall, hepatic injury, blood dyscrasias.
- Rare and serious
- SJS/TEN, DRESS, anaphylaxis, agranulocytosis, severe hepatitis, interstitial nephritis.
- Special groups
- Higher reaction rates in HIV; hemolysis risk in G6PD deficiency.
Common effects
Gastrointestinal. Nausea, anorexia, abdominal discomfort, occasionally vomiting. Usually mild; often resolves with continued use or lower doses.
Maculopapular rash. The most common allergic-type effect of sulfa antibiotics. Often appears between the seventh and fourteenth day of a course, sometimes after the course is finished. Symptoms covers the appearance.
Photosensitivity. Sun-exposed skin reacts more strongly than usual to ultraviolet light. Recognised with sulfa antibiotics and notable with HCTZ. Routine sun precautions reduce risk. More on photosensitivity.
Headache, fatigue. Reported but often hard to separate from the underlying illness.
Less common but important
Hyperkalemia (TMP-SMX). Trimethoprim has structural similarity to amiloride and blocks epithelial sodium channels in the distal nephron, raising serum potassium. The effect is dose-related and is more pronounced in patients with reduced kidney function or on other potassium-sparing drugs (ACE inhibitors, ARBs, spironolactone). Routine prescribing in older patients sometimes includes a check of potassium and creatinine after a few days of therapy.
Elevated creatinine without true GFR fall. Trimethoprim competitively inhibits creatinine secretion in the proximal tubule. Serum creatinine rises modestly; the underlying glomerular filtration rate is unchanged. The rise reverses on stopping the drug. Kidney effects covers this.
Hyponatremia. Reported in older patients, with thiazides and with TMP-SMX. Usually mild but occasionally clinically significant.
Hepatic injury. Mild transaminase elevations are not unusual and often clinically silent. More serious hepatocellular damage is uncommon but reported, particularly in DRESS.
Blood dyscrasias. Leukopenia and thrombocytopenia can occur with sulfa antibiotics, more often than is widely appreciated, particularly on prolonged or high-dose therapy. Megaloblastic anemia is folate-related and more common on long courses; folinic acid is sometimes co-prescribed in high-dose long-course use. Agranulocytosis and aplastic anemia are rare but recognised.
Hemolysis. In G6PD deficiency, oxidative stress from sulfa drugs (notably dapsone and to a lesser extent sulfasalazine) can precipitate hemolytic anemia.
Aseptic meningitis. Rare reaction reported with TMP-SMX. Presents with headache, neck stiffness, fever; resolves on stopping the drug.
Rare and serious
Stevens-Johnson syndrome and toxic epidermal necrolysis. Severe cutaneous adverse reactions with significant mortality. Sulfa antibiotics are among the recognised triggers. Onset typically within four weeks of starting the drug.
DRESS — drug reaction with eosinophilia and systemic symptoms. Onset two to eight weeks in. Rash, fever, lymphadenopathy, eosinophilia, organ involvement.
Anaphylaxis. Uncommon but documented with sulfa antibiotics. IgE-mediated, fast onset, requires emergency management.
Severe drug-induced liver injury. Including rare fulminant hepatic failure.
Interstitial nephritis. An immune-mediated kidney injury, sometimes with fever, rash, and eosinophilia. Kidney effects covers it.
Class-specific effects in non-antibiotic sulfonamides
The non-antibiotic sulfonamides each have their own side effect profile, mostly determined by their pharmacology rather than by the sulfonamide group:
Loop diuretics (furosemide, others) — electrolyte loss, volume depletion, ototoxicity at high doses, hyperuricemia, hyperglycemia.
Thiazides (HCTZ, others) — hypokalemia, hyponatremia, hyperuricemia, mild hyperglycemia, photosensitivity, possible long-term increased risk of non-melanoma skin cancer.
Sulfonylureas — hypoglycemia (the dominant concern), weight gain.
Carbonic anhydrase inhibitors — paresthesias, metabolic acidosis, kidney stones, fatigue.
Celecoxib — NSAID-class effects: cardiovascular risk, gastrointestinal effects (lower than non-selective NSAIDs), kidney effects, fluid retention, hypertension.
Each is covered on its own page: furosemide, hydrochlorothiazide, sulfonylureas, carbonic anhydrase inhibitors, celecoxib.
What is not a side effect of sulfa drugs
A few things are commonly attributed to sulfa drugs but should not be:
An adverse reaction to wine, dried fruit, or processed foods that contain "sulfites" is not a sulfa drug reaction. Sulfites in food and wine covers it; the disambiguation page covers all four "sulf-" terms.
Reactions to sulfur-containing topical preparations (acne soaps, scabies treatments) are local and unrelated to the sulfa drug family.
Magnesium sulfate, used in obstetrics and emergency medicine, is a sulfate, not a sulfa drug. It is not implicated by a sulfa allergy label.