Each of the three main types of diuretic medicines works a little differently, but they all lower the amount of salt and water in the body, which helps to lower blood pressure. The choice of diuretic depends on the condition for which it is being prescribed. Urine flow usually increases within hours of the first dose, but diuretics may take several weeks to treat conditions like high blood pressure. The three main types of diuretics are:

Thiazide diuretics, which treat the vast majority of patients with high blood pressure and are the most commonly prescribed diuretic for heart patients. Thiazides increase urine excretion moderately and are the only type of diuretic that also acts to widen the blood vessels, which also helps to lower blood pressure.
Loop diuretics, which remove a large amount of sodium from the kidneys, produce the greatest increase in urine flow and are more powerful than thiazides. They are often used for patients with congestive heart failure and are also especially useful in emergencies. Although most commonly used as an oral medication, they may be administered intravenously in the hospital to treat patients with severe fluid overload.
Potassium-sparing diuretics, which don't have the side effect of lowering the body's potassium level, which is a problem with thiazide and loop diuretics. These are used frequently in congestive heart failure patients and are often prescribed in conjunction with the other two types of diuretics.

Side Effects of Diuretics
The most common side effect associated with diuretics is an increased elimination of potassium, resulting in a dangerously low level of potassium in the body. With the exception of potassium-sparing versions, all diuretics may cause a loss of potassium, which, if left untreated, increases the risk for heart rhythm disturbances that can be serious. Taking a potassium supplement or eating high-potassium foods (such as bananas and orange juice) may help maintain healthy potassium levels. A potential side effect of potassium-sparing diuretics is a dangerously high level of potassium in people who already have a high potassium level or who have kidney disease. Other potential side effects of diuretics include:

Frequent urination. Usually lasting for up to six hours, this is the most common side effect of diuretics.
Dehydration. Thirst, dry mouth, dizziness, constipation and dark-colored urine may result.
Cramps. Low potassium levels can cause cramps.
Dizziness and weakness. Older people may be particularly susceptible to this side effect, but it often resolves after taking the medication for a while.
Loss of appetite and vomiting. This side effect may also be related to low levels of potassium.

Generally, older patients may be more susceptible to side effects of diuretics and may require lower doses and close observation, but they are routinely prescribed these medications.

A good number of patients taking diuretics develop a resistance to them. We'll discuss diuretic resistance on the next page.