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Discontinued Warning IconPlease Note: This Brand Name drug is no longer available in the US.
(Generic versions may still be available.)

Ismelin Patient Information Including Side Effects

Brand Names: Ismelin

Generic Name: guanethidine (Pronunciation: gwha NETH ih deen)

What is guanethidine (Ismelin)?

Guanethidine lowers blood pressure by decreasing the levels of certain chemicals in the blood. This allows your blood vessels (veins and arteries) to relax (widen).

Guanethidine is used to treat hypertension (high blood pressure). Guanethidine is also used to treat high blood pressure in the kidneys.

Guanethidine may also be used for purposes other than those listed in this medication guide.

What are the possible side effects of guanethidine (Ismelin)?

If you experience any of the following serious side effects, stop taking guanethidine and seek emergency medical attention or contact your doctor immediately:

  • an allergic reaction (difficulty breathing; closing of the throat; swelling of the lips, tongue, or face; or hives);
  • irregular heartbeats;
  • heart failure (shortness of breath, swelling of ankles or legs, sudden weight gain of 5 pounds or more); or
  • chest pain.

Other, less serious side effects are more likely to occur. Continue to take guanethidine and talk to your doctor if you experience

  • unusual fatigue or drowsiness;
  • dizziness (avoid standing up to quickly and use caution when performing hazardous activities);
  • anxiety, depression, or nightmares;
  • diarrhea, nausea, or vomiting (take guanethidine with food or milk if it upsets your stomach);
  • stuffy nose or a dry mouth (sucking on ice chips or sugarless hard candy may relieve a dry mouth);
  • blurred vision;
  • weight gain; or
  • impotence or difficulty ejaculating.

Side effects other than those listed here may also occur. Talk to your doctor about any side effect that seems unusual or that is especially bothersome.

Read the Ismelin (guanethidine monosulfate) Side Effects Center for a complete guide to possible side effects

What is the most important information I should know about guanethidine (Ismelin)?

Use caution when driving, operating machinery, or performing other hazardous activities. Guanethidine may cause dizziness or drowsiness. If you experience dizziness or drowsiness, avoid these activities.

Use caution when rising from a sitting or lying position, especially first thing in the morning. Dizziness may occur while taking guanethidine and may result in a fall.

Do not stop taking guanethidine suddenly without first talking to your doctor. This could cause severely high blood pressure, nervousness, and anxiety.

Before having surgery, tell your doctor or dentist that you are taking this medication.

Side Effects Centers

Report Problems to the Food and Drug Administration


You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.


Get tips on handling your hypertension.