Milk has a lower pH and protein binding capacity than plasma, so drugs distribute into milk depending on their lipophilicity and plasma protein binding. Highly plasma protein-bound drugs remain in the bloodstream and have lower concentrations in milk. All antithyroid drugs are secreted in milk, so breastfeeding is not recommended for women taking these medications. The extent a drug transfers to milk is affected by its pKa, protein binding, and lipophilicity. Drugs that are basic, have low plasma protein binding, and are lipophilic will have higher milk concentrations. The timing of breastfeeding should be considered when taking certain drugs.

1.  Milk differs from plasma in that it has lower pH, lower protein
binding capacity and higher concentration of lipids
 All drugs distribute into milk (having lower pH) depending on
lipophilicity and plasma protein binding
 High plasma protein bound drugs are held in blood and lesser
amounts appear in milk
 All antithyroid drugs are secreted in milk so milk feeding is
not recommended for these women (maternal
hyperthyroidism)

2.  Following characteristics will affect the extent of drug transfer:
◦ pKa - determines the fraction of drug that will be ionized at given pH.
◦ For basic drugs, a greater fraction will be ionized at acidic pH values, so milk
will tend to trap weak bases
◦ Acidic drugs are more ionized at higher pH, so trapped in maternal plasma
◦ Protein binding – highly plasma protein bound drugs → trapped in maternal
plasma because of fewer proteins in milk
◦ Lipophilicity - highly lipophilic drugs will dissolved in lipid content of the
milk
◦ So a basic drug with low plasma protein binding and relative lipophilicity will
have higher concentration in milk (e.g. Sotalol)
◦ If drug is given as a single shot, like sumatriptan for migraine attack, she
should discontinue breast feeding
◦ If chronically as in epilepsy, then it is very important discontinue breast
feeding

3.  Amiodarone, carbimazole, isoniazid, lithium, metronidazole,
propylthiouracil, theophylline, phenobarbitone are avoided
during lactation
 Maximum concentration in milk occurs at a similar time to
the maximum concentration in plasma, so avoid feeding at
this time
 Start dose just after feeding or take the dose & immediately
feed
 Breast feeding should be avoided if infant has G-6-PD
deficiency and if mother is taking aspirin, chlorpromazine,
quinine, quinolones and sulphonamides

4.  Oestrogens decreases prolactin receptors → decreased milk
production, so use progesterone only contraceptives instead
of combined ones
 Dopamine agonists as bromocriptine decreases milk
production
 Thiazide diuretics also decreases milk prodution