The cephalosporins differ from penicillins in the structure of the bicyclic ring system. Cephalexin has a D-phenylglycyl group as substituent at the 7-amino position and an unsubstituted methyl group at the 3-position. Each capsule contains cephalexin monohydrate equivalent to 250 mg, 500 mg or 750 mg of cephalexin.The capsules also contain cellulose, D & C Yellow No. 10, F D & C Blue No. 1, F D & C Yellow No. 6, gelatin, magnesium stearate, silicone, titanium dioxide, and other inactive ingredients.all your doctor at once if you have any of these serious side effects: 1  diarrhea that is watery or bloody; 2  seizure (convulsions); 3  fever, sore throat, and headache with a severe blistering, peeling, and red skin rash; 4  pale or yellowed skin, dark colored urine, fever, confusion or weakness;