Ab initio calculations were performed to determine the effect of 2p and 3p impurities on the structural, electronic, and magnetic properties of cementite. We predict that phosphorus, sulfur, and all 2p impurities replace carbon, while aluminum and silicon substitute for iron. the magnetization and magnetic moments on iron atoms in the special and general positions only slightly change for impurities in the carbon sites, but they essentially decrease for impurities that substitute for iron (Al, Si). We find that boron and nitrogen (to a lesser degree) promote the stabilization of cementite and may be dissolved with the formation of ternary Fe3(C, B) and Fe3(C, N) cementites. Aluminum and especially silicon sharply suppress the formation of cementite.