An important aspect of medical treatment is the effect the treatment will have on the fetus or newborn when given to a pregnant or lactating animal. Many drugs are capable of crossing the placenta and affecting the fetus. Certain antibiotic drugs are toxic to a fetus, while others may affect developing cartilage, bones, and teeth. Some antifungal drugs and all cancer chemotherapeutic drugs are potentially harmful to a developing fetus. Glucocorticoids may cause cleft palate or other defects in puppies.
When medications are given to lactating animals, the excretion of the drug or its byproducts in milk and the possible effects on the suckling newborn must be taken into consideration.