What Are Sulfonamides? Uses, Warnings, Side Effects, and More

Sulfonamides, or “sulfa drugs,” are a group of medicines used to treat bacterial infections. They come in different formulations and may be taken as an oral, topical, vaginal, or ophthalmic (eye) medicine, according to Merck Manual.

“Sulfa drugs” were the first antibacterial agents, and they paved the way for the widespread use of antibiotics, notes Science History Institute. The first sulfonamide, Prontosil, was tested in the 1930s.

Eventually, bacteria started to develop resistance to sulfonamides, and penicillin wound up replacing sulfa drugs as a first-line treatment, according to the National Institutes of Health. While antibiotic resistance is still an issue with sulfa drugs, the medications are still commonly used to treat a variety of bacterial infections.

Sulfa drugs work by inhibiting an enzyme called dihydropteroate synthase (DHPS). DHPS helps produce folic acid, which bacteria need in order to proliferate. If the enzyme is inhibited, then bacteria can’t grow or multiply.

Sulfa drugs may be prescribed to treat urinary tract infections (UTIs); inflammatory bowel disease; malaria; skin, vaginal, and eye infections; burns; and other conditions, notes Merck Manual.

The drugs won't work for viral infections, such as a cold or the flu, per Mayo Clinic.

According to Mayo Clinic, common sulfonamides include:

• sulfasalazine (Azulfidine, Azulfidine EN-tabs)
• acetazolamide xr Diamox Sequels)
• acetyl sulfisoxazole pediatric suspension (Gantrisin)
• sulfisoxazole (Trixazole)
• zonisamide (Zonegran)

Tell your doctor about any medical conditions you have — especially kidney, liver, or blood disorders — before taking sulfonamides.

Sulfonamides may cause blood problems, especially if they're taken for a long period of time, per Mayo Clinic. These blood problems can lead to an increased chance of infections, slow healing, and bleeding of the gums. Be extra careful when using toothbrushes, dental floss, and toothpicks, and postpone dental work until your blood counts have returned to normal.

Tell your doctor if your symptoms don't improve after a few days or get worse after you start taking sulfonamides.

Sulfonamides may make your skin more sensitive to the sun. Exposure to sunlight, even for a brief period of time, may cause rash, itching, redness or other discoloration of the skin, or a severe sunburn. When taking these medications, avoid unnecessary exposure to the sun, and wear sunscreen and protective clothing while you are outside.

Sulfonamides may make you dizzy, so avoid driving, operating machinery, or performing activities that require you to be alert while you are taking these medications.

Your doctor will want to carefully monitor your body's response to this medicine with frequent observation. Be sure to keep all appointments with your doctor's office and laboratory.

Let your doctor know that you're taking a sulfonamide before having any type of medical procedure, including a dental exam or procedure.

Sulfonamides shouldn't be given to infants under 2 months old unless directed by the baby’s doctor.

Older adults are particularly sensitive to the effects of sulfonamides, especially if they are taking diuretics (water pills) along with this medication.

Sulfonamides and Pregnancy

Animal studies have shown that sulfonamides may cause birth defects.

Tell your doctor if you're pregnant, or might become pregnant, before taking any of these drugs.

These medicines can also pass into breast milk. Don't breastfeed while taking sulfonamides.

Allergies to Sulfonamides

Sulfonamides can trigger an allergic reaction.

Tell your doctor if you have ever had any unusual or allergic reaction to sulfonamides or any other medications. Also tell your doctor if you have any other types of allergies, such as to food dyes, preservatives, or animals.

According to Mayo Clinic, common side effects of sulfonamides may include:

• Itching
• Skin rash
• Increased sensitivity of the skin to sunlight
• Diarrhea
• Dizziness
• Headache
• Loss of appetite
• Nausea or vomiting
• Fatigue

Other less common side effects may include:

• Aching of joints and muscles
• Difficulty in swallowing
• Pale skin
• Redness, blistering, peeling, or loosening of skin
• Sore throat and fever
• Unusual bleeding or bruising
• Unusual tiredness or weakness
• Yellow eyes or skin

Rare side effects may include:

• Abdominal or stomach cramps and pain, which may be severe
• Abdominal tenderness
• Blood in urine
• Diarrhea, which may also be bloody and severe
• Greatly increased or decreased frequency of urination or amount of urine
• Increased thirst
• Lower back pain
• Mood or mental changes
• Pain or burning during urination
• Swelling of the front part of the neck

When you are taking sulfonamides, it is very important that your doctor knows about any other medications you are taking, because an interaction can occur.

If you are taking sulfonamides, you should avoid the following medications, according to Mayo Clinic:

• dofetilide
• ketorolac
• levomethadyl
• methenamine

There is a long list of other medications that are not usually recommended to take when you are on sulfonamides (although in some cases they might be necessary). This list includes amoxicillin, chloroquine, erythromycin, ibuprofen, methotrexate, and salicylic acid.