Which antibiotic should not be given in pregnancy?

October 18, 2025 •

4 min read

An estimated 20-60% of pregnant women are prescribed an antibiotic at some point during their pregnancy, making it crucial to know which antibiotic should not be given in pregnancy to protect both maternal and fetal health. While many antibiotics are considered safe, certain drug classes carry significant risks and must be avoided.

Quick Summary

Certain antibiotics, such as tetracyclines and fluoroquinolones, are generally contraindicated during pregnancy due to fetal risks. Other medications, like trimethoprim-sulfamethoxazole, must be avoided during specific trimesters because of potential harm. Infection treatment requires a careful risk-benefit assessment by a healthcare provider.

Key Points

Tetracyclines are absolutely contraindicated: Antibiotics in this class, including doxycycline, are known to cause permanent discoloration of fetal teeth and can affect bone growth if used after the fourth month of pregnancy.
Trimethoprim-Sulfamethoxazole is trimester-specific: This medication (Bactrim) must be avoided in the first trimester due to folic acid interference and later in the third trimester due to kernicterus risk.
Fluoroquinolones require caution: Due to theoretical risks of cartilage damage and mixed human study results, fluoroquinolones like ciprofloxacin are generally not first-line treatments during pregnancy.
Always consult a doctor: Never self-medicate during pregnancy; discuss any infection and potential antibiotic use with your healthcare provider to perform a proper risk-benefit assessment.
Safe alternatives are available: Many common infections can be treated effectively with generally safe antibiotics such as penicillins (amoxicillin) and cephalosporins (cephalexin).

The Importance of Antibiotic Safety in Pregnancy

For a pregnant individual, bacterial infections pose a dual threat: harm to the mother and potential complications for the developing fetus. Treating the infection is essential, but the choice of medication requires careful consideration. A healthcare provider must weigh the benefits of treatment against the risks of a medication that could cross the placenta and cause harm. This is particularly important because fetal organs are most susceptible to damage during the first trimester (the organogenesis period), though risks can exist throughout pregnancy.

Antibiotics Absolutely Contraindicated in Pregnancy

Some antibiotics are widely recognized as unsafe for pregnant individuals and are generally avoided at all stages of gestation. Their known risks to fetal development make them poor choices unless there are no other viable options for a serious, life-threatening infection.

Tetracyclines

The tetracycline class of antibiotics, which includes doxycycline, minocycline, and oxytetracycline, is a primary example of a group of drugs that should not be used in pregnancy.

Fetal Effects: These drugs can bind to calcium in the fetus's developing bones and teeth. Exposure during the second and third trimesters can lead to permanent discoloration (yellow-gray-brown) of the baby's teeth. While bone growth effects are often reversible after discontinuation, the dental staining is permanent.
Maternal Effects: In rare cases, high-dose intravenous tetracycline has been linked to maternal liver failure.
Other Risks: Doxycycline exposure has also been associated with spontaneous abortion in some studies, though more data is needed.

Fluoroquinolones

Fluoroquinolones, which include ciprofloxacin, levofloxacin, and moxifloxacin, are also generally avoided. Concerns stem from animal studies showing potential for cartilage damage, although human data has shown mixed results.

Theoretical Risks: The primary concern with fluoroquinolones is the potential for affecting fetal bone and cartilage development. This is a theoretical risk based on animal studies, and most human studies haven't found a significant link to major malformations.
Recent Findings: Some studies have suggested associations between first-trimester fluoroquinolone exposure and an increased risk of atopic diseases like dermatitis and asthma in children.
Clinical Practice: Due to the availability of safer alternatives, these drugs are not typically first-line treatments during pregnancy.

Chloramphenicol

Chloramphenicol is another antibiotic that is contraindicated in pregnancy. It carries a severe risk of causing a potentially fatal condition in newborns called “Gray baby syndrome,” characterized by a gray discoloration of the skin, low blood pressure, and shock.

Antibiotics to Avoid in Specific Trimesters

Some antibiotics are not safe throughout the entire pregnancy, with risks concentrated during specific trimesters.

Trimethoprim-Sulfamethoxazole (Bactrim)

This combination antibiotic is a common treatment for urinary tract infections but should be avoided at two key times during pregnancy.

First Trimester: Trimethoprim is an antifolate drug that can interfere with folic acid metabolism. Folic acid is critical for proper neural tube development in the first trimester, and interference can increase the risk of neural tube defects, oral clefts, and cardiovascular abnormalities.
Third Trimester: Sulfamethoxazole can displace bilirubin from albumin, increasing the risk of kernicterus (severe jaundice and potential brain damage) in the newborn if used near term.

Nitrofurantoin

Nitrofurantoin (Macrobid) is commonly used for UTIs but has conflicting safety data. While often avoided in the first trimester by some guidelines due to older studies suggesting a potential risk of birth defects, more recent large-scale studies have not found a significant association compared to beta-lactams. However, it should be avoided at term (38-42 weeks of gestation) due to the risk of hemolytic anemia in newborns with G6PD deficiency.

Comparison of Antibiotic Classes in Pregnancy


Antibiotic Class	Examples	Pregnancy Risk	Key Concern/Reason for Avoidance
Tetracyclines	Doxycycline, Minocycline	Contraindicated	Permanent teeth discoloration, bone growth impairment, maternal liver toxicity
Fluoroquinolones	Ciprofloxacin, Levofloxacin	Avoid if possible	Theoretical cartilage damage, potential links to atopic diseases
Chloramphenicol		Contraindicated	Gray baby syndrome
Trimethoprim-Sulfamethoxazole	Bactrim	Trimester-specific	Folic acid interference in 1st trimester; kernicterus in 3rd trimester
Nitrofurantoin	Macrobid	Trimester-specific	Potential 1st trimester risks; hemolytic anemia at term
Macrolides	Erythromycin, Clarithromycin	Cautious use	Conflicting data on malformation risk; azithromycin is safer
Penicillins	Amoxicillin, Ampicillin	Generally Safe	Well-studied and first-line therapy
Cephalosporins	Cephalexin	Generally Safe	Well-studied, often used if penicillin allergy

What to Do if You Were Exposed to an Unsafe Antibiotic

If you discovered you were pregnant after already taking an antibiotic considered unsafe, do not panic. The most important step is to contact your healthcare provider or a maternal-fetal medicine specialist immediately. They can assess the timing and dose of the exposure to determine the risk and develop a monitoring plan for you and the fetus. In many cases, early exposure may not result in harm, but professional guidance is essential.

Conclusion

The selection of antibiotics during pregnancy is a nuanced process guided by established safety data and a careful consideration of the risks and benefits. Certain drug classes, like tetracyclines and chloramphenicol, are strictly off-limits due to known fetal harm. Others, such as trimethoprim-sulfamethoxazole and nitrofurantoin, require avoiding use during specific trimesters. In contrast, penicillins and cephalosporins are generally considered safe and are often the preferred options. A pregnant individual should never self-medicate and must always consult with their healthcare provider to ensure the safest and most effective treatment plan. The ultimate goal is to eliminate the maternal infection with the least possible risk to the developing baby. For more detailed clinical information on drug safety in pregnancy, a resource like MotherToBaby provides evidence-based guidance for patients and healthcare professionals.

Frequently Asked Questions

While no medication is risk-free, several classes of antibiotics, notably penicillins (like amoxicillin) and most cephalosporins (like cephalexin), have a long history of safe use and are considered first-line options for treating infections during pregnancy.

If you took a tetracycline before realizing you were pregnant, contact your doctor immediately. The greatest risk of tooth discoloration occurs after the fourth month of pregnancy, but any exposure should be discussed with a healthcare provider.

Yes, factors such as the timing and dose of the antibiotic, the severity of the infection being treated, and individual patient differences can all play a role. Your healthcare provider will choose the safest medicine at the lowest effective dose for the shortest duration.

The safety of topical antibiotics depends on the specific drug and the extent of absorption into the bloodstream. Many are considered safe due to minimal systemic absorption, but it is always best to consult with your doctor before using any medication, including topical ones, while pregnant.

Bactrim is not recommended in the first trimester because trimethoprim can block the effects of folic acid. This is crucial for preventing neural tube defects in the early stages of fetal development.

In cases of severe infections resistant to first-line agents, a doctor will perform a detailed risk-benefit analysis. A potentially higher-risk antibiotic might be necessary if the untreated infection poses a greater danger to the mother and fetus than the medication itself.

This depends on the specific antibiotic and whether you plan to breastfeed. Some drugs can pass into breast milk and affect the baby, while others are safe. You should discuss all medication plans with your doctor and lactation consultant.