Pregnancy category C is an FDA drug classification meaning “risk cannot be ruled out.” It was assigned to medications where animal studies showed harmful effects on a developing fetus, but no adequate studies had been conducted in pregnant women. The key distinction: the potential benefits of taking the drug may still outweigh those risks. Category C was by far the most common classification, with the vast majority of new medications landing in this category.
What Category C Actually Means
The FDA’s old letter system ranked drugs from A (safest in pregnancy) through X (known to cause birth defects, never use). Category C sat in the middle, covering two distinct scenarios. In the first, animal reproduction studies revealed adverse effects on the fetus, but no one had conducted controlled studies in pregnant women to confirm whether the same thing happens in humans. In the second, no animal studies had been done at all, and there were also no human studies. Either way, the result was the same label: we don’t have enough evidence to say this drug is safe, but we can’t confirm it’s dangerous either.
The types of harm seen in animal studies for category C drugs were often relatively subtle. In typical examples, animals given high doses of a drug during pregnancy showed lower fetal body weight and delayed bone development, but no actual birth defects. The doses used in these animal studies were often 20 to 40 times the recommended human dose, which makes the results hard to translate to real-world use.
Why So Many Drugs Fell Into This Category
More than 90 percent of new medications were classified as either category C, D, or X, and the vast majority were category C. The reason is straightforward: it’s unethical to run clinical drug trials on pregnant women, so nearly every medication hits the market without controlled human pregnancy data. Unless a drug had strong animal safety data (which would push it toward category B) or clear evidence of human harm (category D or X), it defaulted to category C. This made the label frustratingly broad. A drug with minor effects in animal studies at extreme doses carried the same letter as one with no data at all.
How Doctors Weigh the Risks
Seeing “category C” on a prescription doesn’t automatically mean you should stop taking it. Some women enter pregnancy with conditions like asthma, epilepsy, or high blood pressure that require ongoing medication. Stopping treatment can pose a greater risk to both mother and baby than continuing a category C drug. New medical problems can also develop during pregnancy, or existing ones like migraines can worsen, making medication necessary.
The decision comes down to a risk-benefit calculation specific to your situation. A category C drug prescribed for a life-threatening condition carries a very different weight than one prescribed for mild discomfort. Clinicians also tend to favor older medications that have been in use for decades, because years of real-world use in pregnant patients provide informal safety data even when formal studies were never conducted. A drug prescribed to millions of women over 30 years with no pattern of birth defects offers more reassurance than a brand-new medication with the same category C label.
The FDA Replaced Letter Categories in 2015
The FDA published the Pregnancy and Lactation Labeling Rule (PLLR) in December 2014, and it took effect on June 30, 2015. This rule eliminated the A, B, C, D, and X letter system entirely. The agency recognized that a single letter oversimplified complex risk information and often caused confusion. Category C, in particular, was so broad that it told patients and providers very little.
The new system requires three descriptive subsections on drug labels instead of a letter grade. The Pregnancy subsection provides detailed information about known risks and includes a pregnancy exposure registry when one exists, so doctors and patients can report outcomes. The Lactation subsection covers whether the drug passes into breast milk and what effects it might have on a nursing infant. A third subsection, Females and Males of Reproductive Potential, addresses whether pregnancy testing is needed before starting the drug, contraception recommendations, and any effects on fertility.
Drugs approved before June 2015 are being updated gradually, so you may still encounter the old letter categories on some labels or in older reference materials. When you do see “category C,” it helps to know what it originally meant: not a red flag, not a green light, but a gray zone where the decision depends on your specific health needs and available alternatives.