Hi thanks for asking in Healthcare Magic. Diovan is Valsartan and HCTZ is Hydrochlorothiazide. Valsartan is an Angiotensin receptor blocker. Losartan also belongs to the same category. Hydrochlorothiazide is a diuretic. A diuretic increases excretion of both water and electrolytes and results in depletion of electrolytes when given for a long time. All these drugs are given to reduce high BP. Hydrochlorothiazide results in hypokalemia (loss of potassium) as a side effect. Similarly, Valsartan and Losartan result in hyperkalemia (excess of potassium) as their side effect. Both excess or loss of potassium will affect the heart. So the combination of both these group of drugs is advantageous as one will compensate the side effect of the other. Although Valsartan is costlier than Losartan, both are equally effective in controlling BP. Besides, food does not reduce the absorption of Losartan whereas food interferes with the absorption of Valsartan. This is the reason for better bio availability (amount that reaches the blood) for Losartan (33%) than Valsartan (23%). Both drugs have similar side effects profile. So, Losartan is a better drug than Valsartan and is also cost effective. I hope this is useful to you.