How effective is simvastatin?

Very: If you are asking about how well it works for lowering cholesterol, then it is very effective

Efficacy: Studies show when evaluating the efficacy of any drug one must consider the disease treated, hyperlipidemia, and any benefits and side effects of the drug treating it (Statin). Statins have been shown in studies to decrease your bad cholesterol, increase your good cholesterol, and have been shown to decrease mortality from Cardiovascular disease. My fave is Rosuvastatin- least drug interactions.