Inhibitory effects of some drugs on glucose 6-phosphate dehydrogenase from the erythrocytes of human have been investigated. For this purpose, at the beginning, erythrocyte glucose 6-phosphate dehydrogenase was purified 2256 times in a yield of 44.22% by using ammonium sulphate precipitation and 2', 5'-ADP Sepharose 4B affinity gel. Temperature of +4°C was maintained during the purification process. Enzyme activity was determined with the Beutler method by using a spectrophotometer at 340 nm. This method was utilized for all kinetic studies. Ketotifen, dacarbazine, thiocolchicoside, meloxicam, methotrexate, furosemide, olanzapine, methylprednizolone acetate, paricalcitol, ritodrine hydrochloride, and gadobenate-dimeglumine were used as drugs. All the drugs indicated the inhibitory effects on the enzyme. Ki constants for glucose 6-phosphate dehydrogenase were found by means of Lineweaver-Burk graphs. While methylprednizolone acetate showed competitive inhibition, the others displayed non-competitive inhibition. In addition, IC(50) values of the drugs were determined by plotting Activity% vs [I].