Sulfanilamide, the original sulfa drug, carries a range of hazards that span from predictable side effects like kidney crystal formation to rare but life-threatening skin reactions and a historical mass-poisoning event that killed over a hundred people and reshaped drug regulation worldwide. Though sulfanilamide itself is rarely prescribed today, it belongs to a broader class of sulfonamide antibiotics that remain in wide clinical use, and the hazards first identified with sulfanilamide still inform how these drugs are monitored and prescribed.
The 1937 Disaster That Defined Sulfanilamide’s Reputation
Sulfanilamide arrived in the 1930s as something close to a miracle. It was the first commercially available antimicrobial drug, and it worked against streptococcal infections that had been routinely fatal. But the tragedy that cemented sulfanilamide in public memory had nothing to do with the drug itself. In 1937, a pharmaceutical company called S.E. Massengill formulated a liquid version of sulfanilamide for patients who could not swallow tablets. To dissolve the drug, the company’s chemist chose diethylene glycol, a sweet-tasting solvent that is chemically related to antifreeze. No safety testing was done on the final product before it shipped.
The preparation, marketed as “Elixir Sulfanilamide,” killed at least 105 people, many of them children who were given the raspberry-flavored liquid for sore throats and ear infections.1PubMed. Elixirs, diluents, and the passage of the 1938 Federal Food, Drug and Cosmetic Act Laboratory analysis showed the preparation was roughly a 10 percent sulfanilamide solution in about 72 percent diethylene glycol, with some coloring and flavoring agents.2JAMA. Pathologic Effects of Elixir of Sulfanilamide (Diethylene Glycol) Poisoning Diethylene glycol is metabolized in the body to oxalic acid and other toxic compounds that destroy the kidneys, and the deaths were caused by this solvent rather than by sulfanilamide itself.
Under the law at the time, Massengill could only be charged with mislabeling the product as an “elixir” (which technically implies an alcohol-based solution). The company paid a fine. The public outcry, however, was enormous and directly led to the passage of the 1938 Federal Food, Drug, and Cosmetic Act, which for the first time required drug manufacturers to demonstrate safety before selling a new product.3PubMed. Elixirs, diluents, and the passage of the 1938 Federal Food, Drug and Cosmetic Act Every drug approval process in the United States traces back to that sulfanilamide-adjacent catastrophe.
How Sulfanilamide Works, and Why That Creates Problems
Sulfanilamide kills bacteria by starving them of folate, a B vitamin they need to build DNA and grow. Bacteria synthesize their own folate from a molecule called para-aminobenzoic acid (PABA). Sulfanilamide looks enough like PABA that the bacterial enzyme responsible for this step, dihydropteroate synthase, grabs the drug instead of the real building block. The result is a dead-end product the bacterium cannot use, and without folate, it stops dividing.4Journal of Hazardous Materials Advances. Sulfonamides repress cell division in the root apical meristem by inhibiting folates synthesis This mechanism is shared across all sulfonamide antibiotics.5PubMed Central. Microbiome-generated antifolates
Human cells do not make their own folate; we get it from food. That difference is what makes sulfonamides selective against bacteria rather than our own tissues. But “selective” does not mean “harmless.” Sulfanilamide and its relatives interact with other enzymes in the body, including carbonic anhydrases, which play roles in kidney function, fluid balance, and acid-base regulation.6PubMed. Sulfa and trimethoprim-like drugs – antimetabolites acting as carbonic anhydrase, dihydropteroate synthase and dihydrofolate reductase inhibitors These off-target effects help explain why sulfonamides cause a wider array of side effects than their elegant antimicrobial mechanism might suggest.
Kidney Damage from Crystal Formation
One of the oldest and best-understood hazards of sulfonamide drugs is crystalluria, the formation of drug crystals inside the kidneys. Sulfanilamide and related compounds are not always fully soluble in urine, especially when urine is concentrated or acidic. The drug and its metabolites can precipitate into crystals within the kidney tubules, physically blocking the tiny channels that carry urine. This obstruction can cause acute kidney injury.
The risk is highest with certain sulfonamides like sulfadiazine, which is still used to treat toxoplasmosis in people with HIV. In those patients, high doses combined with dehydration create ideal conditions for crystal formation. Urine sediment examined under polarized light can reveal characteristic yellow-brown rosette-shaped sulfonamide crystals, and volume depletion is a major predisposing factor.7PubMed Central. Sulfadiazine-induced crystalline nephropathy: a case report of a reversible cause of acute kidney injury in advanced HIV The good news is that this form of kidney injury is often reversible with hydration and dose adjustment. But clinicians have to be aware it can happen, and patients on sulfonamides are routinely advised to drink plenty of fluids, particularly if they are on higher doses or have any degree of dehydration.
Older sulfonamides like sulfanilamide were especially prone to this because they were less soluble than the newer drugs that replaced them. This is one reason sulfanilamide fell out of clinical use in most settings: successor drugs like sulfamethoxazole offered better solubility, and therefore a lower risk of kidney crystals, though the hazard was never fully eliminated from the drug class.
Severe Skin Reactions
Sulfonamide antibiotics carry one of the highest drug-class risks for Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), two related conditions in which the skin essentially blisters and peels off in sheets. These are rare but can be fatal. In a large study examining medication-related risk, sulfonamide antibiotics like trimethoprim-sulfamethoxazole were associated with a crude relative risk of 172 for developing SJS or TEN, compared to people not taking any of the high-risk drugs.8The New England Journal of Medicine. Medication use and the risk of Stevens-Johnson syndrome or toxic epidermal necrolysis That number dwarfed the risk associated with other drug classes in the same study: aminopenicillins had a relative risk around 7, quinolones around 10, and cephalosporins around 14.
SJS and TEN are thought to be immune-mediated reactions, meaning the body’s own immune system attacks the skin in response to the drug or its metabolites. The reactions typically begin one to three weeks after starting the medication and can progress rapidly from a rash to widespread skin detachment. Because sulfonamides sit at the top of the risk profile for these conditions, prescribers generally warn patients to stop the drug immediately and seek medical attention if they develop a new rash, mouth sores, or blistering while on a sulfonamide antibiotic. Catching it early matters: the mortality rate for TEN can exceed 30 percent, while milder SJS cases have a much better prognosis when the causative drug is withdrawn quickly.
Risks in Newborns and Late Pregnancy
Sulfonamides pose a specific and serious hazard to newborns. The concern is kernicterus, a form of brain damage caused by excessive bilirubin crossing into the brain. Bilirubin is a yellow pigment produced when the body breaks down red blood cells, and it normally circulates bound to a protein called albumin. Sulfonamides can displace bilirubin from albumin, leaving free bilirubin to rise in the blood. In adults, this displacement is usually harmless because the liver clears bilirubin efficiently. In newborns, the liver is immature and cannot keep up. The unbound bilirubin can cross the blood-brain barrier and damage neurons.9PubMed Central. Cotrimoxazole and neonatal kernicterus: a review
This hazard is well established enough that sulfonamide antibiotics are generally avoided in the final weeks of pregnancy and in infants during their first weeks of life. The risk applies to the drug reaching the baby either through the placenta before birth or through breast milk after delivery. Kernicterus can cause permanent neurological damage including hearing loss, movement disorders, and intellectual disability, making this one of the most consequential sulfonamide hazards in terms of lifelong impact when it occurs.
The “Sulfa Allergy” Misconception
Walk into any doctor’s office and you will see “sulfa allergy” listed prominently on patient charts. It is one of the most commonly reported drug allergies. But the clinical picture around sulfa allergies is muddier than most patients realize, particularly when it comes to the widespread belief that having a sulfa allergy means you must avoid all drugs containing a sulfonamide chemical group.
Many non-antibiotic medications contain a sulfonamide-like group in their molecular structure: certain diuretics (like furosemide and hydrochlorothiazide), some diabetes medications, and the migraine drug sumatriptan, among others. For years, patients with sulfonamide antibiotic allergies were warned away from all of these. More recent evidence suggests that advice was overcautious. A comprehensive review of sulfonamide hypersensitivity found that cross-reactivity between sulfonamide antibiotics and non-antibiotic sulfonamides has not been conclusively established, because the two categories of drugs do not share the same chemical accessory groups that are thought to trigger immune reactions.10PubMed. A Comprehensive Review of Sulfonamide Hypersensitivity: Implications for Clinical Practice
A large study published in the New England Journal of Medicine sharpened this point further. The researchers tracked nearly a thousand patients who had experienced an allergic reaction to a sulfonamide antibiotic and then later received a sulfonamide non-antibiotic. About 10 percent of them did have an allergic reaction to the non-antibiotic sulfonamide. That sounds like cross-reactivity until you look at the control: patients who had a sulfonamide antibiotic allergy were actually more likely to react to penicillin, a drug with no structural relationship to sulfonamides at all. The risk of reacting to a non-antibiotic sulfonamide was lower than the risk of reacting to penicillin in the same patients.11PubMed. Absence of cross-reactivity between sulfonamide antibiotics and sulfonamide nonantibiotics The conclusion: people with sulfa antibiotic allergies appear to have a general predisposition to drug allergies rather than a specific chemical cross-reactivity with the sulfonamide group.
This matters practically because withholding useful medications like diuretics from patients with a “sulfa allergy” label can lead to worse health outcomes, particularly in heart failure or hypertension management. The allergist community has gradually shifted toward recommending that most patients with a documented sulfonamide antibiotic allergy can safely receive non-antibiotic sulfonamides, ideally with appropriate monitoring.
Drug Metabolism and Individual Variation
Sulfanilamide is processed in the liver primarily through a reaction called acetylation, where an acetyl group is tacked onto the drug molecule. The resulting metabolite, acetylated sulfanilamide, is then excreted by the kidneys.12PubMed. Sulfonamide acetylation in isolated rat liver cells This sounds straightforward, but the acetylated form of sulfanilamide is actually less soluble than the parent drug, which is part of why it tends to form crystals in the kidneys.
People differ significantly in how fast they acetylate drugs. This genetic variation in a family of enzymes called N-acetyltransferases divides the population roughly into fast and slow acetylators. Slow acetylators keep higher levels of the active drug in their blood for longer, which can be therapeutically useful but also raises the risk of dose-dependent side effects. Fast acetylators clear the drug more quickly but produce more of the less-soluble acetylated metabolite, potentially increasing the risk of crystalluria. Animal studies have confirmed that changes in liver function directly alter how quickly acetylated sulfanilamide appears in the blood.13PubMed. Pharmacokinetics of the antimicrobial drug Sulfanilamide is altered in a preclinical model of vascular calcification The practical upshot is that the same dose of a sulfonamide can behave quite differently in two people, and kidney and liver function both influence how likely any given patient is to experience toxicity.
Drug Interactions Worth Knowing About
In modern clinical practice, sulfanilamide itself is rarely prescribed, but its descendant sulfamethoxazole, almost always given in combination with trimethoprim (as the combination product co-trimoxazole), is one of the most widely used antibiotics in the world. Sulfamethoxazole blocks the same enzyme sulfanilamide does, while trimethoprim blocks a second enzyme further along in the folate pathway, creating a sequential blockade that is far more effective than either drug alone.14ScienceDirect (Mayo Clinic Proceedings). Symposium on Antimicrobial Agents Trimethoprim-Sulfamethoxazole This synergy is clinically valuable, but the combination also broadens the hazard profile.
Sulfonamides interact with several other common medications. They can increase the blood-thinning effect of warfarin by displacing it from protein-binding sites, raising the risk of bleeding. They can enhance the blood-sugar-lowering effect of certain diabetes drugs, risking dangerous hypoglycemia. They may increase blood levels of methotrexate, a drug used for autoimmune conditions and cancer, by competing for kidney excretion. And the potassium-sparing effect of trimethoprim, when combined with ACE inhibitors or potassium supplements, can push potassium levels dangerously high, particularly in patients with reduced kidney function. These interactions do not occur with sulfanilamide specifically, since it has fallen out of use, but they apply to the sulfonamide class that sulfanilamide fathered.
Environmental Hazards and Antimicrobial Resistance
Sulfanilamide hazards extend beyond the patient taking the drug. Sulfonamide antibiotics are among the most frequently detected pharmaceutical contaminants in rivers, lakes, and agricultural runoff. They enter the environment through wastewater treatment plants that cannot fully remove them, and through agricultural use, since sulfonamides are widely used in livestock and aquaculture.
In aquatic environments, even low concentrations of sulfonamides affect non-target organisms. Research has shown that nearly all sulfonamides are toxic to algae, with the mechanism mirroring their antibacterial action: they inhibit folate synthesis in plant cells, which also rely on this pathway.15PubMed Central. A review on the ecotoxicological effect of sulphonamides on aquatic organisms Experimental work in freshwater biofilms found that environmentally relevant concentrations of sulfonamides doubled diatom mortality and caused physical deformities in diatom cells, something that had not been documented before for this drug class.16PubMed Central. Environmental Concentrations of Sulfonamides Can Alter Bacterial Structure and Induce Diatom Deformities in Freshwater Biofilm Communities Diatoms are single-celled algae that form the base of many aquatic food webs, so disrupting them can ripple through an ecosystem.
The resistance dimension is equally troubling. Sulfonamide-resistant bacteria are widespread in environments influenced by wastewater discharge. Research on fish ponds in agricultural regions found that the bacterium Acinetobacter accounted for about 35 percent of sulfonamide-resistant strains, and similar resistance patterns have been documented in rivers affected by sewage treatment plants across multiple countries.17PubMed Central. A review on the ecotoxicological effect of sulphonamides on aquatic organisms One study tracking what happens downstream of a wastewater treatment plant found that sulfonamide resistance genes persisted at least 13 kilometers downstream, even when the actual drug concentrations in the water were vanishingly low. The researchers concluded that limiting the release of resistance-gene-carrying bacteria from treatment plants may matter more than reducing the trace amounts of drug in the water.18PubMed Central. The fate of sulfonamide resistance genes and anthropogenic pollution marker intI1 after discharge of wastewater into a pristine river stream
Monitoring sulfonamides in the environment requires sensitive analytical methods. High-performance liquid chromatography and UV spectroscopy are the most widely used screening techniques for detecting sulfonamide residues in water, soil, and food products.19Current Analytical Chemistry. Sulphonamide: Distribution, Toxicology, Environmental Characteristics, and Analysis – A Review These methods allow regulators to track drug concentrations in the parts-per-billion range, which is the level at which ecological effects start to appear. The challenge is that sulfonamides are produced in enormous volumes globally, and wastewater treatment technology has not kept pace with the scale of contamination.
Why Sulfanilamide Still Matters
Sulfanilamide is not a drug most people will ever take. It was eclipsed decades ago by better-tolerated descendants, and the formulation that caused the 1937 disaster would be unthinkable under modern drug regulation. Yet the drug’s legacy is layered into every sulfonamide prescription written today. The kidney crystal risk, the severe skin reactions, the neonatal hazard, and the allergy profile all trace back to the same basic molecular family. When a pharmacist flags a “sulfa allergy” on your chart, or when an environmental scientist measures antibiotic residues in a stream, the thread leads back to this simple molecule. The hazards first catalogued with sulfanilamide became the template for understanding an entire drug class, and that template is still being updated as new ecological and genetic evidence comes in.