G6PDH is an enzyme of the pentose phosphate pathway that catalyzes the reaction below:

Glucose-6-Phosphate +  NADP+ <=> 6-Phosphogluconolactone + NADPH

Individuals whose G6PDH is partly inactive are prone to oxidative stress. This arises because NADPH produced by the pentose phosphate pathway is required for reduction of glutathione, which, in turn, helps reduce peroxides. In the absence of sufficient NADPH, glutathione remains oxidized and peroxides cannot be neutralized.