Alpha blockers —

These drugs may cause fast heart rate, dizziness or a drop in blood pressure when you stand up i.e postural hypoteson

Combined alpha and beta blockers — People taking these drugs may experience a drop in blood pressure when they stand up.

Central agonists — Alpha methyldopa (Aldomet) may produce a greater drop in blood pressure when you're in an upright position (standing or walking) and may make you feel weak or faint if the pressure has been lowered too far. This drug may also cause drowsiness or sluggishness, dryness of the mouth, fever or anemia. Male patients may experience impotence. If this side effect persists, your doctor may have to change the drug dosage or use another medication.

Clonidine (Catapres), guanabenz (Wytensin) or guanfacine (Tenex) may produce severe dryness of the mouth, constipation or drowsiness. If you're taking any of these drugs, don’t stop suddenly, because your blood pressure may rise quickly to dangerously high levels.

Peripheral adrenergic inhibitors — Reserpine may cause a stuffy nose, diarrhea or heartburn. These effects aren't severe and no treatment is required other than to change the amount of drugs taken. If you have nightmares or insomnia or get depressed, tell your doctor. You should stop using the drugs.

Guanadrel (Hylorel) or guanethidine (Ismelin) may cause some diarrhea, which may persist in some people. This side effect usually becomes less of a problem if you continue treatment.

These drugs reduce blood pressure more when you stand. Consequently, you may get dizzy and lightheaded and feel weak when you get out of bed in the morning or stand up suddenly. If you notice any of these reactions — and if they persist for more than a minute or two —  sit or lie down and either reduce or omit the next dose of the drug. If symptoms continue, contact your doctor.

When you're taking guanethidine, don't keep standing in the hot sun or at a social gathering if you begin to feel faint or weak. These activities cause low blood pressure. Male patients may experience impotence. Contact your doctor if this occurs. These drugs are rarely used unless other medications don’t help.

Blood vessel dilators — Hydralzine (Apresoline) may cause headaches, swelling around the eyes, heart palpitations or aches and pains in the joints. Usually none of these symptoms are severe, and most will go away after a few weeks of treatment. Minoxidil (Loniten) is a potent drug that's usually used only in resistant cases of severe high blood pressure. It may cause fluid retention (marked weight gain) or excessive hair growth.

NOTE:  If you're taking any of the medications discussed here, don't stop taking them without consulting your doctor. You may not find all adverse effects in all indviduals.