Breast feeding provides a situation in which the suckling infant may become the unintentional recipient of medications taken by nursing mother. At first glance the problem might appear to be of some importance, since most drugs pass into breast milk to a varying extent. In addition, the young baby has several attributes that make him unduly susceptible to the effects of drugs. He has a low body weight. His kidneys and liver are immature, and thus his ability to eliminate drugs is generally less than his mother and accumulation may easily occur. Evidence of early toxicity may be masked by the obvious lack of symptomatology and the difficulty in interpreting nonspecific complaining behavior.

There may also be difficulty in attributing non specific sign to drug effect, especially if the sign are not representative of the same overdose state in older child or adult. On rare occasions the child’s genetic makeup may produce a deficiency in his ability to metabolize some drugs in the same manner as the mother.

As with any dug effect, the overwhelmingly important aspects are the amount of drug consumed and the sensitivity of the individual. The amount of drug received by the baby is clearly dependent on the concentration of drug in milk and the amount of milk consumed. Which is usually 500-700 ml daily. The concentration of drug in milk depends on its transfer from plasma, which is the result of the influence of molecular weight, of fat and water solubility, of pKa, of protein binding, and of concentration gradient of the drug at the alveolar interface between plasma and milk.

The pH of milk is acidic with respect to plasma. Drugs that are fat soluble and are not bound to plasma protein diffuse passively into milk with concentration gradient. Basic drugs such as antihistamines, theophylline, meperidine and opium alkaloids, imipramine, amitriptyline, isoniazid, erythromycin, and lincomycin tend to achieve concentrations in milk equal to or greater than unbound concentration in plasma.