What is a category B antibiotic? Understanding Pregnancy Safety Ratings

October 11, 2025 •

2 min read

Before 2015, the U.S. Food and Drug Administration (FDA) utilized a letter-based system to classify a drug's potential risks during pregnancy. Under this old framework, a significant number of commonly prescribed antibiotics were classified as Category B, a designation that provided guidance for healthcare providers and expectant mothers regarding medication safety.

Quick Summary

A Category B antibiotic was a medication where animal reproduction studies showed no risk, or animal risks were not confirmed in human studies. It was a classification primarily used for guiding antibiotic choices in pregnant patients before being phased out by the FDA.

Key Points

Former FDA System: Category B is part of the letter-based pregnancy risk system (A, B, C, D, X) used before 2015 but still referenced.
Definition: An antibiotic is classified as Category B if animal studies showed no fetal risk but human data is lacking, or if animal studies showed risk not confirmed in human studies.
Common Examples: Many prescribed antibiotics like amoxicillin, cephalexin, and azithromycin were considered Category B.
Risk vs. Benefit: Category B indicated a relatively low risk, and the benefits of treating infection often outweighed risks.
Modern Labeling: The FDA now uses the PLLR, a narrative approach providing detailed risk information.
Clinical Guidance: Antibiotic choice in pregnancy involves a careful risk-benefit analysis considering infection, gestational stage, and patient factors.

The Significance of FDA Pregnancy Categories

For decades, the FDA's letter-based categorization system (A, B, C, D, X) was the standard for assessing a drug's potential risks during pregnancy. While this system has been replaced by the more detailed Pregnancy and Lactation Labeling Rule (PLLR), the older categories are still frequently referenced in clinical practice and legacy literature. An understanding of the former Category B helps explain why some medications were considered relatively safe for pregnant women, despite the lack of extensive human trial data.

Defining a Category B Antibiotic

An antibiotic classified as Category B involved scenarios where either animal studies showed no harm but human studies were insufficient, or animal studies showed adverse effects that were not confirmed in human studies. The Category B rating suggested a lower risk compared to medications in Categories C, D, or X. Deciding to use any medication during pregnancy involves weighing the potential benefits against risks.

Common Examples of Category B Antibiotics

Many common antibiotics were formerly classified as Category B, such as Penicillins (Amoxicillin, Ampicillin), Cephalosporins (Cephalexin, Cefdinir), Azithromycin, and Metronidazole. Penicillins and cephalosporins have a long history of safe use in pregnancy.

The Transition from Old to New FDA Labeling

In 2015, the FDA introduced the Pregnancy and Lactation Labeling Rule (PLLR), replacing the letter system with more detailed narrative information based on available data. Despite this change, the older categories are still widely referenced.

How Clinical Decisions are Made

Healthcare providers prescribing antibiotics during pregnancy assess the risks and benefits, using the former Category B status as one data point indicating a history of use without widespread reports of fetal harm. Decisions also depend on the specific infection, gestational age, and individual patient factors. Treating bacterial infections is essential during pregnancy, and Category B antibiotics are often used.

Comparison of FDA Pregnancy Categories A, B, and C

Here is a comparison of the old FDA categories for pregnancy risk:


Feature	Category A	Category B	Category C
Animal Studies	No fetal risk.	May show no risk, or risk not confirmed in human studies.	Adverse effect on fetus.
Human Studies	Adequate, controlled studies show no risk.	Adequate, controlled human studies are lacking.	No adequate, controlled human studies.
Interpretation	Safest.	Generally safe, but human data limited.	Risk cannot be ruled out; benefits may outweigh risks.
Examples (Antibiotics)	Few examples; Mycostatin mentioned in some sources.	Penicillins, Cephalosporins, Azithromycin.	Fluoroquinolones (e.g., Ciprofloxacin).

Conclusion

The former Category B antibiotic designation remains a reference in pharmacology and clinical practice, indicating medications with a low likelihood of fetal harm based on available data, even if controlled studies in pregnant women are limited. Any antibiotic use during pregnancy requires a thorough, individualized risk-benefit assessment. The new PLLR offers a more detailed approach to this decision. For current FDA labeling information, consult the official FDA website regarding the PLLR.

Frequently Asked Questions

Many common antibiotics fall under the former Category B, including penicillins like amoxicillin, cephalosporins like cephalexin, and some macrolides such as azithromycin.

No, a Category B rating does not guarantee 100% safety. It indicates that, based on available animal studies and limited human data, there is no evidence of harm, but conclusive, controlled studies in pregnant women are often lacking.

The main difference is the evidence of risk. For Category B, animal studies typically show no fetal risk (or an unconfirmed risk), while for Category C, animal studies have shown an adverse effect, and human data is insufficient.

The old system is still referenced because it was the standard for decades and is ingrained in medical literature and clinical practice. While the FDA has replaced it with a more detailed system, many clinicians continue to use the familiar categories as a quick reference.

No, a pregnant woman should not stop taking a prescribed antibiotic without consulting her healthcare provider. The provider has already performed a risk-benefit analysis, and stopping the medication could lead to serious complications from the untreated infection.

The classification is informed by animal reproduction studies and, in some cases, limited human observational data, especially from the first trimester, to determine the potential for fetal harm.

The new PLLR moves away from a single letter grade. Instead, it requires a more comprehensive and detailed description of a medication's potential risks and benefits during pregnancy and lactation based on all available data.