Drug compatibility and breastfeeding

Drug compatibility and breastfeeding

On drug compatibility and breastfeeding, today we share with you the main points of the practical workshop on how to use the e-lactancia.org website that the pharmacist and co-author of the website Silvia Moyano gave at the LactApp Medical Congress.

Moyano addressed, among other aspects, the pharmacokinetic characteristics of a substance to be able to assess the compatibility of drugs and breastfeeding and the risk posed by that substance for an infant. He explained how to take into account the process of the passage of a substance into breast milk, evaluating the substances that, based on their characteristics, determine whether or not they pose a risk to the infant. These are the steps to be evaluated:

These are the steps to be evaluated:

  1. Passage of the substance into maternal blood circulation. This is the first step for the product to reach the breast milk. In order to be secreted into the milk, the drug must be absorbed and it must reach the blood circulation Therefore, products with null oral bioavailability, topical application, inhaled drugs, otic or ophthalmologic products have  a lower risk since they are not distributed through the bloodstream.


  1.  Specific characteristics of the drug itself to pass into the milk . Drugs that reach the breast milk in smaller quantities are those that:

– Have a molecular weight greater than 1000 daltons.

– Have a percentage of binding to plasma proteins greater than 85%.

– Have a basic pH (milk is more acid than blood).

– Have a low degree of lipophilicity

Variable Units Description More possibility of compatibility if: 
PM daltons Molecular Weight >800-1000 daltons
Prot. bond % Plasma protein binding >85%
Vd L/Kg Volume of distribution >12L o >0,2 L/Kg
pKa Molecule acidity <7
Lipophilic low lipophilic
  1.  Excretion of the product by breast milk, which is assessed with the following parameters:

– Milk/plasma index (L/P): the ratio of the concentration of the product in milk to the concentration of the product in blood plasma. When the value is less than 1, it indicates that the product has less tendency to pass into milk from the blood, therefore it is a safer product than others with a value greater than 1.

– Theoretical dose: this is the amount of substance that reaches the infant, considering an intake of 150ml/kg/24h of milk. The lower the theoretical dose, the more compatible the product is.

– Relative dose: this is the absolute dose, relative to the dose taken by the mother. If the value is less than 10%, it is considered compatible with lactation.

– Pediatric relative dose: It compares the dose received by the infant through breast milk with the dose that would be received if the infant were treated (orally or parenterally) with that product. If the value is less than 10%, it is considered compatible with breastfeeding.

With regard to relative doses, there is an exception: when the products in question may have serious effects on the infant, in which case, even if the relative doses are less than 10%, they are not considered compatible with breastfeeding.


Variable Units Description More possibility of compatibility if: 
L/P ratio Concentration in milk/concentration in mother’s plasma <1
Theoretical dose mg/Kg/d Amount of substance received by the infant per day, considering an intake of 150 mL/Kg/d Low
Relative dose % % of the maternal dose reaching the infant through the milk < 10%
Pediatric relative dose % % dose reaching the infant as a proportion of the usual dose in pediatrics <10%


  1. Intestinal absorption by the infant, therefore the following products should be mentioned:

– Products that are not absorbed orally (no oral bioavailability), that is, that the mother has received them parenterally.

– Products that are destroyed by the digestive system (e.g. omeprazole).

  1. The last point to assess in each product is whether it is harmful to the infant, since it may be a product that reaches the milk in large quantities but has no adverse effects on the infant and therefore it would be compatible for the woman to take it during the breastfeeding period.


Reference on drug compatibility and breastfeeding

APILAM. In: e-lactancia.org. APILAM: Association for the promotion and scientific and cultural research on breastfeeding; 2002 updated 07 Jun 2022; accessed 08 Jun 2022. Available at https://www.e-lactancia.org/


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