Understanding Medication Safety Ratings

Why Safety Ratings Matter

Medication use during pregnancy and breastfeeding requires careful consideration. Virtually every substance that enters the maternal bloodstream can, to varying degrees, cross the placenta or appear in breast milk. Safety ratings help healthcare providers make informed, evidence-based decisions and have transparent discussions with patients.

The Cybele classification system is adapted from the widely used international framework and reflects the available scientific evidence for each medication.

Pregnancy Safety Categories

Every medication in the Cybele database is assigned one of the following pregnancy safety categories based on the best available evidence from human and animal studies:

Category	Label	What it means
A	Generally Safe	Adequate, well-controlled human studies show no increased risk of foetal abnormalities. These medications are considered safe to use during pregnancy.
B	Probably Safe	Animal studies show no evidence of harm and there are no adequate human studies, or animal studies showed adverse effects that were not confirmed in adequate human studies. Most commonly used medications fall in this category.
C	Use with Caution	Animal studies have shown adverse foetal effects and there are no adequate human studies, or no animal or human studies exist. The potential benefit may warrant use in pregnant women despite potential risks. Use only if clearly needed.
D	Positive Risk Evidence	There is positive evidence of human foetal risk based on adverse reaction reports or studies. However, potential benefits may warrant use in pregnant women in serious situations (e.g., life-threatening conditions) where safer alternatives are not available.
X	Contraindicated	Studies in animals or humans have demonstrated foetal abnormalities, or there is positive evidence of foetal risk based on adverse reaction reports. The risks involved clearly outweigh potential benefits. These medications are contraindicated in women who are or may become pregnant.

Breastfeeding Safety

In addition to pregnancy categories, Cybele provides breastfeeding-specific safety information for each medication where evidence is available. Breastfeeding safety is assessed on a separate scale based on:

• The amount of the medication that passes into breast milk (relative infant dose, or RID)
• Potential effects on milk production
• Known effects on the infant
• The age and health of the infant

As a general rule, a relative infant dose below 10% of the weight-adjusted maternal dose is considered acceptable for most medications, but clinical context always matters.

Important Limitations

Safety categories provide a useful overview, but they are not a substitute for clinical judgement. Always consider:

• The gestational age and trimester of exposure
• The dose and duration of treatment
• The severity and urgency of the condition being treated
• Whether safer alternatives exist
• Individual patient factors (comorbidities, other medications, etc.)

When in doubt, consult a specialist in clinical pharmacology, maternal-foetal medicine, or contact your regional teratology information service.