HCTZ is hydrochlorothiazide. THe thiazides were the first drug used to combat HBP in the 1950's...before that people with numbers in the 200's had to rely on phenobarbitol and MUCH sedation (FDR had to put up with this.)
They work at the most basic level of BP control...dump excess sodium from the body (and the blooodstream) via the kidneys. Since death ensues with a VERY teeny change in sodium concentration, the body IMMEDIATELY compensates by dumping an appropriate amount of water to keep balance . Less water- Less pressure.
Only problems come because thiazides dump and save a whole cascade of other ions (charged molecules of various blood components) to preserve electriacl balance.
Prime concerns are an increase in Uric Acid leves (gout) and a decrease in potassium. MANY other chemicals are affected but these two are of the most concern.
If you OD on the thiazides you COULD get excess sodium depletion and dehydration...this is the only DEADLY consequence ...one should learn the symptoms of depleted sodium, potassium and check the uric acid levels periodically.
The government has recently concluded that diuretics are perhaps the BEST way to control BP and it's only drug company greed and doctor's stupidity and lack of care for patients that has all these EXPENSIVE alternatives rammed down our gullets.
(FDA actually said first line should be cheap thiazides and cheap beta blockers...doctors HATE the word "cheap."