In plain words
The relative infant dose compares the amount of a medicine a breastfed baby may receive through milk with the amount the parent takes, both adjusted for weight. It is expressed as a percentage. Many clinicians use a rough benchmark of 10 percent, below which exposure is often considered acceptable for many drugs. That benchmark is a rule of thumb, not a safety line, and it does not fit every drug, especially those with a narrow safe range. Newborns and preterm babies clear medicines more slowly, so the same percentage can matter more for them. This number is one input. A lactation pharmacist or a reference such as can help you read it for a specific medicine. Your provider can weigh that against the condition being treated.
The deeper version
Summary exposure metrics for drugs in milk include the milk-to-plasma concentration ratio and the relative infant dose (RID), the infant's weight-adjusted dose via milk expressed as a percentage of the maternal weight-adjusted dose. Bennett (1996) popularized the 10 percent benchmark as a conventional threshold below which exposure is generally considered acceptable for many drugs. Reference databases such as LactMed use it as one input. It is a heuristic, not a safety threshold, and it fails for drugs with narrow therapeutic windows, for drugs with active metabolites, and for neonates with immature clearance. Controversy centers on the lack of pharmacokinetic data for many drugs and on how to combine exposure estimates with adverse-event data, which are sparse in breastfed infants.
Where it comes up
- Getting sick while postpartumPostpartum: Your Body
- Medications and breastfeeding: how to checkFeeding Your Baby
- Therapy and medication after birth: what the options arePostpartum: Mind
What mothers here say about relative infant dose
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Sources
- Bennett PN (ed.) — Drugs and Human Lactation, 2nd edition (1996)
- U.S. National Library of Medicine — LactMed: Drugs and Lactation Database