The disposition of many medications is altered during pregnancy. Due to changes in many physiological parameters as well as variability in the activity of maternal drug-metabolizing enzymes, the efficacy and toxicity of drugs used by pregnant women can be difficult to predict. Enzymatic activity exhibited by the placenta and fetus may affect maternal drug distribution and clearance also. In addition, efflux transporters have been detected in high amounts within placental tissue, potentially limiting fetal exposure to xenobiotics. Dosage adjustments of antiepileptic drugs, antidepressants and anti-infectives administered during pregnancy have been required due to these changes in drug metabolism and disposition. As such, pregnant women may require different dosing regimens than both men and non-pregnant women.