After oral administration, cycloserine is readily absorbed from the gastrointestinal tract, with peak blood levels occurring in 4 to 8 hours. Blood levels of 25 to 30 μg/mL can generally be maintained with the usual dosage of 250 mg twice a day, although the relationship of plasma levels to dosage is not always consistent. Concentrations in the cerebrospinal fluid, pleural fluid, fetal blood, and mother’s milk approach those found in the serum. Detectable amounts are found in ascitic fluid, bile, sputum, amniotic fluid, and lung and lymph tissues. Approximately 65 percent of a single dose of cycloserine can be recovered in the urine within 72 hours after oral administration. The remaining 35 percent is apparently metabolized to unknown substances. The maximum excretion rate occurs 2 to 6 hours after administration, with 50 percent of the drug eliminated in 12 hours. ^[1]

Adapted from the FDA Package Insert.