Erythromycin is an antibiotic used to treat a wide range of bacterial infections, from strep throat and pneumonia to skin infections and certain sexually transmitted infections. It belongs to the macrolide class of antibiotics and works by slowing down bacterial protein production, which stops bacteria from multiplying. It’s available in oral, topical, and eye ointment forms, making it one of the more versatile antibiotics still in regular use.
Respiratory Infections
Erythromycin is commonly prescribed for mild to moderate upper and lower respiratory tract infections, including sore throats caused by strep bacteria, ear infections, and certain types of pneumonia. It’s particularly useful for pneumonia caused by atypical organisms that don’t respond to standard antibiotics, including Legionnaires’ disease. It also treats whooping cough, both to reduce symptoms and to prevent the infected person from spreading the bacteria to others.
For people who are allergic to penicillin, erythromycin serves as an important alternative for treating strep throat and preventing rheumatic fever, a serious complication that can damage the heart valves if strep infections go untreated.
Skin and Soft Tissue Infections
Erythromycin treats mild to moderate skin infections caused by staph and strep bacteria, including cellulitis, impetigo, and wound infections. It also treats erythrasma, a superficial skin infection that causes reddish-brown patches in skin folds. Topical forms of erythromycin are widely used for acne, where the antibiotic targets the bacteria that contribute to breakouts on the skin’s surface.
Sexually Transmitted and Pelvic Infections
Erythromycin is approved for several sexually transmitted infections. It treats chlamydia infections of the urethra, cervix, and rectum, especially when other first-line antibiotics can’t be used. It’s also an alternative treatment for primary syphilis in people allergic to penicillin, and for pelvic inflammatory disease caused by gonorrhea when penicillin isn’t an option.
Pregnant women with chlamydia infections may be prescribed erythromycin because some antibiotics commonly used for chlamydia are unsafe during pregnancy.
Eye Ointment for Newborns
One of erythromycin’s most widespread uses has nothing to do with treating an active infection. A thin strip of 0.5% erythromycin ointment is applied to both eyes of nearly every newborn in the United States within 24 hours of birth. This prevents a potentially blinding eye infection that can occur if the baby is exposed to gonorrhea or chlamydia bacteria during delivery. The CDC and the U.S. Preventive Services Task Force both recommend this prophylaxis, and erythromycin is the only ophthalmic ointment currently recommended for this purpose. The ointment is applied regardless of whether the baby is delivered vaginally or by cesarean section.
Gastroparesis and Stomach Motility
Beyond its antibiotic properties, erythromycin has a notable off-label use: it stimulates stomach contractions. At lower doses than those used for infections (typically 125 to 250 mg three times a day, compared to the standard antibiotic dose of up to 500 mg four times a day), erythromycin mimics a natural gut hormone that triggers the stomach to push food forward. This makes it useful for gastroparesis, a condition where the stomach empties too slowly, causing nausea, bloating, and vomiting. According to the UK’s National Institute for Health and Care Excellence, erythromycin is one of only a few medicines used to help speed up stomach emptying in this condition.
How Erythromycin Works
Erythromycin doesn’t kill bacteria directly. Instead, it binds to a tunnel inside the bacterial ribosome, the molecular machine that builds proteins. When the antibiotic blocks this tunnel, the ribosome slows dramatically during the protein-building process. Bacteria can still start making proteins, but they can’t finish them efficiently. Without functional proteins, bacteria can’t grow or reproduce, giving the immune system time to clear the infection.
Which Bacteria It Covers
Erythromycin is strongest against gram-positive bacteria, the category that includes strep, staph, and the bacteria behind diphtheria and listeriosis. It also works against several gram-negative species that cause specific diseases: the bacteria responsible for whooping cough, gonorrhea, meningitis, and Legionnaires’ disease. Atypical organisms like chlamydia and mycoplasma, which lack the cell walls that many antibiotics target, also fall within erythromycin’s range.
It does not work against common gut bacteria like E. coli, Salmonella, or Pseudomonas. Resistance is a growing concern, particularly among staph bacteria, where resistance genes can spread between bacterial strains.
Common Side Effects
Stomach problems are the most frequent complaint. Nausea, abdominal cramps, bloating, and diarrhea are common enough that some people have difficulty completing a full course. Ironically, the same property that makes erythromycin useful for gastroparesis (stimulating stomach contractions) is what causes these side effects in people taking it for infections. Taking it with food can reduce stomach upset, though some formulations are absorbed better on an empty stomach.
A more serious but less common concern is the effect on heart rhythm. Erythromycin can prolong a specific electrical interval in the heart (the QT interval), which in rare cases leads to abnormal heartbeats or fainting. People with existing heart rhythm problems or those taking other medications that affect heart rhythm face higher risk.
Drug Interactions to Watch For
Erythromycin is classified by the FDA as a moderate inhibitor of one of the liver’s most important drug-processing enzymes. This means it slows the breakdown of many other medications, potentially causing them to build up to dangerous levels in the body. Cholesterol-lowering statins, certain blood thinners, anti-seizure medications, and sedatives are among the drugs affected. It also increases blood levels of the heart medication digoxin by interfering with its transport out of cells.
If you’re taking other medications, your prescriber will need to check for interactions before starting erythromycin. In some cases, the dose of your other medication may need to be adjusted, or a different antibiotic may be a better choice.