Cycloserine 250mg is a second‑line anti‑tuberculosis antibiotic used as part of combination regimens for multidrug‑resistant or extensively drug‑resistant tuberculosis, never as monotherapy, due to rapid resistance and the need for synergistic activity with other TB drugs. It inhibits mycobacterial cell‑wall synthesis by blocking alanine racemase and D‑alanine:D‑alanine ligase, suppressing bacterial growth.