Any EC 1.1.1.* (oxidoreductase acting on donor CH-OH group, NAD+ or NADP+ acceptor) inhibitor that inhibits HMG-CoA reductases. Hydroxymethylglutaryl-CoA reductase inhibitors have been shown to lower directly cholesterol synthesis. The Enzyme Commission designation is EC 1.1.1.34 for the NADPH-dependent enzyme and EC 1.1.1.88 for an NADH-dependent enzyme.