Erythromycin is generally considered safe during pregnancy and has long been one of the preferred antibiotics for pregnant women who are allergic to penicillin. The FDA classified it as Pregnancy Category B, meaning animal studies showed no harm to the fetus and no adequate controlled studies exist in humans. That said, the safety picture has some important nuances depending on which form of erythromycin you’re taking, when during pregnancy you take it, and whether it’s oral or topical.
What the Birth Defect Data Shows
The biggest concern most people have is whether an antibiotic could cause birth defects when taken in the first trimester, the period when a baby’s organs are forming. A large French nationwide cohort study compared first-trimester exposure to macrolide antibiotics (the class erythromycin belongs to) against amoxicillin. After adjusting for other risk factors, macrolide exposure showed no increased risk of major birth defects overall, with a relative risk of 1.00. No elevated risk was found for most individual types of malformations.
The FDA’s own labeling does note that some observational studies have reported cardiovascular malformations after erythromycin exposure in early pregnancy. However, the French cohort study specifically looked for this association and did not confirm it. Two small signals emerged for spina bifida and fused fingers or toes, but after statistical correction for multiple comparisons, neither remained significant. In plain terms, when researchers accounted for the fact that they were testing dozens of possible associations at once, none held up.
Animal studies reinforce this. Pregnant rats and mice given doses roughly one to three times the maximum recommended human dose showed no evidence of birth defects or harm to the embryo.
The One Form to Avoid: Erythromycin Estolate
Erythromycin comes in several salt forms, and one of them, erythromycin estolate, poses a real liver risk during pregnancy. In a controlled trial comparing erythromycin estolate to placebo in pregnant women, about 10% of those taking the estolate form developed abnormally elevated liver enzymes, compared to less than 2% in the placebo group. All six participants with elevated liver enzymes who were further tested also showed abnormal results on a second liver marker, confirming genuine liver stress rather than a lab fluke.
This is not a minor difference. Pregnancy already places extra demands on the liver, and adding a drug that causes liver inflammation in roughly one in ten pregnant users is a significant concern. Other forms of erythromycin, such as erythromycin base and erythromycin ethylsuccinate, do not carry this same liver risk and are the ones typically prescribed during pregnancy.
Possible Link to Pyloric Stenosis
Pyloric stenosis is a condition where a baby’s stomach outlet thickens and narrows, causing forceful vomiting in the first weeks of life. It’s treatable with a minor surgery but still worth knowing about. Some evidence suggests that maternal erythromycin use during pregnancy may modestly increase the risk. One study found a risk ratio of 2.51 for infants developing pyloric stenosis when their mothers received erythromycin after the first prenatal visit, though the confidence interval was wide enough that the result wasn’t statistically definitive.
The absolute risk remains very low. Pyloric stenosis affects roughly 2 to 3 out of every 1,000 births normally. Even if erythromycin doubles that risk, the chance of it affecting your baby is still small. Still, this is one reason doctors typically reserve erythromycin for situations where the benefits clearly outweigh any theoretical risks, rather than prescribing it casually.
Topical Erythromycin Is a Different Story
If you’re using erythromycin cream or gel for acne, the safety profile is more reassuring. A pharmacokinetic study found that while topical erythromycin does produce detectable levels in the blood, the concentrations are very low. In most participants, measurable drug appeared in only a handful of blood samples. Critically, topical application did not meaningfully affect liver enzyme activity, even at higher-than-normal doses. The researchers specifically noted this holds true for vulnerable populations, including pregnant women.
Topical erythromycin is generally considered moderately safe in pregnancy, though some manufacturers still recommend stopping during pregnancy simply because formal trials haven’t been conducted. In practice, many dermatologists continue to recommend it as one of the safer acne treatments for pregnant women, since alternatives like retinoids are clearly harmful to a developing baby.
How Well It Works During Pregnancy
One practical limitation of erythromycin in pregnancy is that it doesn’t cross the placenta efficiently. This matters most for syphilis treatment: erythromycin does not reach the fetus in high enough concentrations to prevent congenital syphilis. The FDA labeling explicitly states that infants born to mothers treated with oral erythromycin for early syphilis should still receive penicillin treatment after birth.
For chlamydia, which is one of the most common reasons erythromycin is prescribed during pregnancy, a randomized trial found a cure rate of 77% with erythromycin compared to 91% with azithromycin. The difference wasn’t statistically significant in that particular study, but the trend favors azithromycin, which is also why many providers now prefer a single dose of azithromycin over a multi-day erythromycin course for chlamydia in pregnancy when both options are available.
Erythromycin’s lower cure rate is partly explained by the longer dosing schedule (typically taken multiple times daily for seven days), which makes it harder to complete the full course, especially when pregnancy-related nausea is already an issue.
Safety While Breastfeeding
If your pregnancy question extends to the postpartum period, erythromycin passes into breast milk at low levels. After a 500 mg oral dose, milk concentrations hover around 1 to 1.2 mg/L. These amounts are considered too low to cause significant effects in a nursing infant, and erythromycin is routinely given directly to newborns for other medical reasons, so the drug itself is well tolerated by infants.
In a follow-up study of 17 breastfed infants whose mothers were taking erythromycin, two experienced diarrhea and two showed irritability. None of these reactions required medical attention. The main things to watch for in your baby are loose stools and signs of yeast overgrowth like thrush or diaper rash, since erythromycin can disrupt normal gut bacteria.