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Sulfonamides and Sulfonamide Combinations

By Dawn Merton Boothe, DVM, PhD, Department of Anatomy, Physiology, and Pharmacology, College of Veterinary Medicine, Auburn University

Sulfonamides are the oldest and remain among the most widely used antibacterial agents in veterinary medicine, chiefly because of low cost and their relative efficacy in some common bacterial diseases. The synergistic action of sulfonamides with specific diaminopyrimidines renders these drugs much more effective than sulfonamides alone.


The many available sulfonamides and sulfonamide derivatives can be categorized into several types, based mainly on their indications and duration of action in the body. Probably the most common classification is based on water versus lipid solubility or duration of effect. Although there are many sulfonamide antimicrobials, only a few are used clinically in animals.

Standard Use Sulfonamides:

In most species, members of this large group are administered 1–4 times/day, depending on the drug, to control systemic infections caused by susceptible bacteria. In some instances, administration of the sulfonamide can be less frequent if the drug is eliminated slowly in the species being treated. Sulfonamides included in this class, depending on the species, are sulfathiazole, sulfamethazine (sulfadimidine), sulfamerazine, sulfadiazine, sulfapyridine, sulfabromomethazine, sulfaethoxypyridazine, sulfamethoxypyridazine, sulfadimethoxine, and sulfachlorpyridazine.

Highly Soluble Sulfonamides Used for Urinary Tract Infections:

A few very water-soluble sulfonamides, eg, sulfisoxazole (sulfafurazole) and sulfasomidine, are rapidly excreted via the urinary tract (>90% in 24 hr) mostly in an unchanged form; because of this, they are primarily used to treat urinary tract infections.

Poorly Soluble Sulfonamides Used for Intestinal Infections:

Some sulfonamide derivatives, such as sulfaguanidine, are so insoluble that they are not absorbed from the GI tract (<5%). Phthalylsulfathiazole and succinylsulfathiazole undergo bacterial hydrolysis in the lower GI tract with the consequent release of active sulfathiazole. Salicylazosulfapyridine (sulfasalazine) is also hydrolyzed in the large intestine to sulfapyridine and 5-aminosalicylic acid, an anti-inflammatory agent that might be used for management of ulcerative colitis in dogs.

Potentiated Sulfonamides:

A group of diaminopyrimidines (trimethoprim, methoprim, ormetoprim, aditoprim, pyrimethamine) inhibit dihydrofolate reductase in bacteria and protozoa far more efficiently than in mammalian cells. Used alone, these agents are not particularly effective against bacteria, and resistance develops rapidly. However, when combined with sulfonamides, a sequential blockade of microbial enzyme systems occurs with bactericidal consequences. Examples of such potentiated sulfonamide preparations include trimethoprim/sulfadiazine (co-trimazine), trimethoprim/sulfamethoxazole (co-trimoxazole), trimethoprim/sulfadoxine (co-trimoxine), and ormetoprim/sulfadimethoxine. Sulfonamides are used in combination with pyrimethamine to treat protozoal diseases such as leishmaniasis and toxoplasmosis. (See also Regulatory Considerations and Drug Withdrawal and Milk Discard Times.)

Topical Sulfonamides:

Several sulfonamides are used topically for specific purposes. Sulfacetamide is not highly efficacious but is occasionally used to treat ophthalmic infections. Mafenide and silver sulfadiazine are used on burn wounds to prevent invasion by many gram-negative and gram-positive organisms. Sulfathiazole is commonly included in wound powders for the same purpose.

General Properties

The sulfonamides are derivatives of sulfanilamide, which is the nucleus common to all. The addition or substitution of various functional groups to the amido group or in which various substitutions on other amino groups result in compounds with varying physical, chemical, pharmacologic, and antibacterial properties. Although amphoteric, sulfonamides generally behave as weak organic acids and are much more soluble in alkaline aqueous solutions than in acidic solutions. Those of therapeutic interest have pKa values of 4.8–8.6. Water-soluble sodium or disodium salts are used for parenteral administration. Such solutions are highly alkaline, somewhat unstable, and readily precipitate with the addition of polyionic electrolytes. In a mixture of sulfonamides (eg, the sulfapyrimidine group), each component drug has its own solubility; therefore, a combination of sulfonamides is more water soluble than a single drug at the same total concentration. This is the basis of triple sulfonamide mixtures used clinically. The N-4 acetylated sulfonamides, except for the sulfapyrimidine group (sulfamethazine, sulfamerazine, sulfadiazine), are less water soluble than their nonacetylated forms. This has bearing in the development of sulfonamide crystalluria. The highly insoluble sulfonamides (phthalylsulfathiazole and succinylsulfathiazole) are retained in the lumen of the GI tract for prolonged periods and are known as “gut-active” sulfonamides. Trimethoprim and ormetoprim are basic drugs.

Antimicrobial Activity

Mode of Action:

The sulfonamides are structural analogues of para-aminobenzoic acid (PABA) and competitively inhibit dihydropterate synthetase, an enzyme that facilitates PABA as a substrate for the synthesis of dihydrofolic acid (folic acid). Dihydrofolate is a precursor for formation of tetrahydrofolate (folinic acid), an essential component of the coenzymes responsible for single carbon metabolism in cells. Sulfonamides are antimetabolites that substitute for PABA, resulting in blockade of several enzymes needed for the biogenesis of purine bases and other metabolic reactions necessary for formation of RNA. Protein synthesis, metabolic processes, and inhibition of growth and replication occur in organisms that cannot use preformed (eg, dietary) folate. The effect is bacteriostatic, although a bactericidal action is evident at the high concentrations that may be found in urine. Diaminopyrimidines such as trimethoprim inhibit dihydrofolate reductase, which is further into the folic acid synthesis pathway. The combination of a sulfonamide and a diaminopyrimidine results in synergistic, bactericidal actions on susceptible organisms; as such, the combination is referred to as a "potentiated" sulfonamide.

The optimal ratio in vitro for the combination of trimethoprim or ormetoprim and a sulfonamide depends on the type of microorganism but is usually ~1:20. However, the commercially available preparations use a ratio of 1:5 because of pharmacokinetic considerations that presumably result in the optimal ratio at the site of infection.

Sulfonamides are most effective in the early stages of acute infections when organisms are rapidly multiplying. They are not active against quiescent bacteria. Typically, there is a latent period before the effects of sulfonamide therapy become evident. This lag period occurs because the bacteria use existing stores of folic acid, folinic acid, purines, thymidine, and amino acids. Once these stores are depleted, bacteriostasis occurs. Bacterial growth can resume when the concentration of PABA increases or when the level of sulfonamide falls below an enzyme-inhibitory concentration. Because of the bacteriostatic nature of sulfonamides, adequate cellular and humoral defense mechanisms are critical for successful sulfonamide therapy when used as sole agents. Even potentiated sulfonamides, which are bactericidal, are time dependent in their antibacterial efficacy.

Although all of the sulfonamides have the same mechanism of action, differences are evident with respect to activity, pharmacokinetic fate, and even antimicrobial spectrum at usual concentrations. The differences are due to the variety of physiochemical characteristics seen among the sulfonamides.

The efficacy of sulfonamides can be reduced radically by excess PABA, folic acid, thymine, purine, methionine, plasma, blood, albumin, tissue autolysates, and endogenous protein-degradation products.

Bacterial Resistance:

Resistance to sulfonamides is both chromosomally and plasmid mediated. Altered proteins such that affinity is reduced appears to be the most common mechanism of resistance. For example, in staphylococci, chromosomally mediated resistance reflects mutations in genes encoding for dihydropterate synthetase and plasmid-mediated resistance reflects mutations in dihydrofolate reductases, with the latter causing high-level resistance to trimethoprim. Staphylococci may have acquired some mechanisms of sulfonamide resistance from enterococci. Because sulfonamides act in a competitive fashion, overproduction of PABA can also preclude inhibition of dihydropterate synthetase. Alternate pathways of folic acid synthesis may also contribute to low-level resistance. Cross-resistance between sulfonamides is common. Resistance emerges gradually and is widespread in many animal populations. Plasmid-mediated sulfonamide resistance in intestinal gram-negative bacteria is often linked with ampicillin and tetracycline resistance.

Antimicrobial Spectra:

The spectrum of all sulfonamides is generally the same. Sulfonamides inhibit both gram-positive and gram-negative bacteria, Nocardia, Actinomyces spp, and some protozoa such as coccidia and Toxoplasma spp. More active sulfonamides may include several species of Streptococcus, Staphylococcus, Salmonella, Pasteurella, and even Escherichia coli in their spectra. Strains of Pseudomonas, Klebsiella, Proteus, Clostridium, and Leptospira spp are most often highly resistant, as are rickettsiae, mycoplasmas, and most Chlamydia.

Pharmacokinetic Features

There are notable differences among the many sulfonamides with respect to their pharmacokinetic fate in the various species. The standard classification of short-, medium-, and long-acting sulfonamides used in human therapeutics is usually inappropriate in veterinary medicine because of species differences in disposition and elimination.


Sulfonamides may be administered PO, IV, IP, IM, intrauterine, or topically, depending on the specific preparation. Except for the poorly absorbed sulfonamides intended for local treatment of intestinal infections, most are rapidly and completely absorbed from the GI tract of monogastric animals. Absorption from the ruminoreticulum is delayed, especially if ruminal stasis is present. Therapeutic doses of sulfonamides are usually administered PO except in acute life-threatening infections when IV infusions are used to establish adequate blood concentrations as rapidly as possible. Sulfonamides are frequently added to drinking water or feed either for therapeutic purposes or to improve feed efficiency. A few highly water-soluble preparations may be injected IM (eg, sodium sulfadimethoxine) or IP (some irritation of the peritoneum can be seen). Absorption is rapid from these parenteral sites. Generally, sulfonamide solutions are too alkaline for routine parenteral use.

Trimethoprim is rapidly absorbed after administration PO (plasma concentrations peak in ~2–4 hr) except in ruminants, in which it tends to be trapped in the ruminoreticulum and appears to undergo a degree of microbial degradation.

Absorption occurs readily from parenteral injection sites; effective antibacterial concentrations are reached in <1 hr, and peak concentrations in ~4 hr.


Sulfonamides are distributed throughout all body tissues. The distribution pattern depends on the ionization state of the sulfonamide, the vascularity of specific tissues, the presence of specific barriers to sulfonamide diffusion, and the fraction of the administered dose bound to plasma proteins. The unbound drug fraction is freely diffusible. Sulfonamides are bound to plasma proteins to a greater or lesser extent, and concentrations in pleural, peritoneal, synovial, and ocular fluids may be 50%–90% of that in blood. Sulfadiazine is ≥90% bound to plasma proteins. Concentrations in the kidneys exceed plasma concentrations, and those in the skin, liver, and lungs are only slightly less than the corresponding plasma concentrations. Concentrations in muscle and bone are ~50% of those in the plasma, and those in the CSF may be 20%–80% of blood concentrations, depending on the particular sulfonamide. Low concentrations are found in adipose tissue. After parenteral administration, sulfamethazine is found in jejunal and colonic contents at about the same concentration as in blood. Passive diffusion into milk also occurs; although the concentrations achieved are usually inadequate to control infections, sulfonamide residues may be detected in milk. Trimethoprim and ormetoprim are basic drugs that tend to accumulate in more acidic environments such as acidic urine, milk, and ruminal fluid.

Trimethoprim diffuses extensively into tissues and body fluids. Tissue concentrations are often higher than the corresponding plasma concentrations, especially in lungs, liver, and kidneys. Approximately 30%–60% of trimethoprim is bound to plasma proteins. The extent of metabolic transformation of trimethoprim has not yet been established, although there is a suggestion that hepatic biotransformation can be extensive, at least in ruminants. This may not be the case in all species; >50% of a dose is excreted unchanged in many instances. Trimethoprim is largely excreted in the urine by glomerular filtration and tubular secretion. A substantial amount may also be found in the feces. Concentrations in milk are often 1–3.5 times higher than those in plasma.


Sulfonamides are usually extensively metabolized, mainly by several oxidative pathways, acetylation, and conjugation with sulfate or glucuronic acid. Species differences are marked in this regard. The acetylated, hydroxylated, and conjugated forms have little antibacterial activity. Acetylation (poorly developed in dogs) reduces the solubility of most sulfonamides except for the sulfapyrimidine group. The hydroxylated and conjugated forms are less likely to precipitate in urine.


Most sulfonamides are excreted primarily in the urine. Bile, feces, milk, and sweat are excretory routes of lesser significance. Glomerular filtration, active tubular secretion, and tubular reabsorption are the main processes involved. The proportion reabsorbed is influenced by the inherent lipid solubility of individual sulfonamides and their metabolites and by urinary pH. Urinary pH, renal clearance, and the concentration and solubility of the respective sulfonamides and their metabolites determine whether solubilities are exceeded and crystals precipitate. This can be prevented by alkalinizing the urine, increasing fluid intake, reducing dose rates in renal insufficiency, and using triple-sulfonamide or sulfonamide-diaminopyrimidine combinations.

Pharmacokinetic Values:

There are great differences between the pharmacokinetic values of various sulfonamides in animals, and extrapolation of these values is rarely appropriate; for example, the plasma half-life of sulfadiazine is 10.1 hr in cattle and 2.9 hr in pigs. The recommended dose rates and frequencies reflect this disparity in elimination kinetics.

The plasma half-life of trimethoprim is quite prolonged in most species; effective concentrations may be maintained for >12 hr, with the result that the frequency of administration is usually 12–24 hr. The elimination rates of trimethoprim in sheep seem to be much shorter than in monogastric species.

Therapeutic Indications and Dose Rates

The sulfonamides are commonly used to treat or prevent acute systemic or local infections. Disease syndromes treated with sulfonamides include actinobacillosis, coccidiosis, mastitis, metritis, colibacillosis, pododermatitis, polyarthritis, respiratory infections, and toxoplasmosis.

Sulfonamides are more effective when administered early in the course of a disease. Chronic infections, particularly with large amounts of exudate or tissue debris present, often are not responsive. In severe infections, the initial dose should be administered IV to reduce the lag time between dose and effect. For drugs with a long elimination half-life, the initial dose should be double the maintenance dose. Adequate drinking water should be available at all times, and urine output monitored. A course of treatment should not exceed 7 days under usual circumstances. If a favorable response is seen within 72 hr, treatment should be continued for 48 hr after remission to prevent relapse and the emergence of resistance. The ability to mount an immune response must be intact for successful sulfonamide therapy.

A selection of general dosages for some sulfonamides is listed in Dosages of Sulfonamides. The dose rate and frequency should be adjusted as needed for the individual animal.

Dosages of Sulfonamides



Dosage, Route, and Frequency



66 mg/kg, PO, tid

Cattle, sheep, pigs

66 mg/kg, PO, every 4 hr



220 mg/kg/day, PO or IV (initial dose; half for subsequent doses)



50 mg/kg, PO, bid



55 mg/kg/day, PO (initial dose; half for subsequent doses)



55 mg/kg/day, PO


110 mg/kg/day, PO (initial dose, half for subsequent doses)



132 mg/kg, PO, bid (initial dose, half for subsequent doses)



160 mg/kg, PO, bid (initial dose; half for subsequent doses)

Dosages of Potentiated Sulfonamides

Combination Sulfonamide

Dosage, Route, and Frequency


15–60 mg/kg/day, PO, IV, or IM


55 mg/kg/day, PO (initial dose; half for subsequent doses)

Special Clinical Concerns

Adverse Effects and Toxicity:

Adverse reactions to sulfonamides may be due to hypersensitivity or direct toxic effects. Possible hypersensitivity reactions include urticaria, angioedema, anaphylaxis, skin rashes, drug fever, polyarthritis, hemolytic anemia, and agranulocytosis. Keratitis sicca is a recognized adverse effect. The allergic response targets, in part, metabolites of the aryl amine of sulfonamides. Because dogs are deficient in acetylation, they may be at risk of increased formation of phase I metabolites associated with adverse effects. Crystalluria with hematuria, and even tubular obstruction, is not common in veterinary medicine. Acute toxic manifestations may be seen after too rapid IV administration or if an excessive dose is injected. Clinical signs include muscle weakness, ataxia, blindness, and collapse. GI disturbances, in addition to nausea and vomiting, may occur when sulfonamide concentrations are sufficiently high in the tract to disturb normal microfloral balance and vitamin B synthesis. Sulfonamides depress the cellulolytic function of ruminal microflora, but the effect is usually transient (unless excessively high concentrations are reached). Several adverse effects have been reported after prolonged treatment, including bone marrow depression (aplastic anemia, granulocytopenia, thrombocytopenia), hepatitis and icterus, peripheral neuritis and myelin degeneration in the spinal cord and peripheral nerves, photosensitization, stomatitis, conjunctivitis, and keratitis sicca. Mild follicular thyroid hyperplasia may be associated with prolonged administration of sulfonamides in sensitive species such as dogs, and reversible hypothyroidism can be induced after treatment with high doses in dogs. Several sulfonamides can lead to decreased egg production and growth. Topically, the sulfonamides retard healing of uncontaminated wounds.

Up to 10 times the recommended dose of trimethoprim has been given with no adverse effects. Prolonged administration of trimethoprim at reasonably high concentrations leads to maturation defects in hematopoiesis due to impaired folinic acid synthesis. This effect is readily reversible by supplementation with folinic acid.


Sulfonamide solutions are incompatible with calcium- or other polyionic-containing fluids as well as many other preparations. Sulfonamides may be displaced from their plasma-protein-binding sites by other acidic drugs with higher binding affinities. Antacids tend to inhibit the GI absorption of sulfonamides. Alkalinization of the urine promotes sulfonamide excretion, and urinary acidification increases the risk of crystalluria. Some sulfonamides act as microsomal enzyme inhibitors, which may lead to toxic manifestations of concurrently administered drugs such as phenytoin.

Effects on Laboratory Tests:

Bilirubin, BUN, bromsulphthalein (BSP®), eosinophils, methemoglobin, AST, and ALT may be increased. Platelet, RBC, and WBC counts are often decreased. Urinalysis may show a change in color, glucose, porphyrins, and urobilinogen. Sulfonamide crystals may also be found.

Regulatory Considerations and Drug Withdrawal and Milk Discard Times:

Sulfonamides are among the drugs for which extra-label use restrictions exist in lactating dairy cattle. Currently allowable drugs are sulfadimethoxine, sulfabromethazine, and sulfathoxypyridazine. In addition, sulfonamide residues, particularly in swine and poultry, continue to be a focus of detection. Because of adverse effects in people, including allergic reactions, attention must be made to withdrawal times. Regulatory requirements for withdrawal times for food animals and milk discard times vary among countries and may change. These must be followed carefully to prevent food residues and consequent public health implications. There are some prohibitions on use of sulfonamides in the USA, including use in dairy cattle. The times listed in Drug Withdrawal and Milk Discard Times of Sulfonamides serve only as general guidelines.

Drug Withdrawal and Milk Discard Times of Sulfonamides



Withdrawal Time (days)

Milk Discard Time (hr)











Triple sulfonamide solutionb








a 28 days for slow-release bolus

b 8% sodium sulfamethazine, 8% sodium sulfapyridine, 8% sodium sulfathiazole

Drug Withdrawal and Milk Discard Times of Potentiated Sulfonamides

Combination Sulfonamide

Withdrawal Time (days)

Milk Discard Time (days)





5 (PO), 28 (parenteral)

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