Guanethidine (patient information)

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Guanethidine is no longer available in the U.S. If you are currently taking guanethidine, you should call your doctor to discuss switching to another treatment.

Guanethidine may cause severe dizziness or fainting, especially in the morning. Hot weather, alcohol, and exercise can increase this effect. Avoid sudden or prolonged standing. To decrease dizziness or faintness, get up slowly from a sitting or lying position. If you feel dizzy or faint at any time, you should lie or sit down.

Why this medication is prescribed

Guanethidine is used to treat high blood pressure. It works by decreasing your heart rate and relaxing the blood vessels so that blood can flow more easily through the body.

This medication is sometimes prescribed for other uses; ask your doctor or pharmacist for more information.

How this medication should be used

Guanethidine comes as a tablet to take by mouth. It is usually taken once a day. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take guanethidine exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor.

Guanethidine controls high blood pressure but does not cure it. Continue to take guanethidine even if you feel well. Do not stop taking guanethidine without talking to your doctor. Abruptly stopping guanethidine may increase blood pressure and cause unwanted side effects.

Special precautions

Before taking guanethidine:

  • tell your doctor and pharmacist if you are allergic to guanethidine or any other drugs.
  • tell your doctor and pharmacist what prescription and nonprescription medications you are taking, especially amitriptyline (Elavil), chlorpromazine (Thorazine), clomipramine (Anafranil), desipramine (Norpramin), diet pills, doxepin (Adepin, Sinequan), ephedrine, fluphenazine (Prolixin), haloperidol (Haldol), imipramine (Tofranil), MAO inhibitors [phenelzine (Nardil) or tranlcypromine (Parnate)], methylphenidate (Ritalin), minoxidil (Loniten), nortriptyline (Aventyl, Pamelor), perphenazine (Trilafon), phenylephrine, phenylpropanolamine, protriptyline (Vivactil), pseudoephedrine, sleeping pills, thioridazine (Mellaril), thiothixine (Navane), trifluoperazine (Stelazine), trimipramine (Surmontil), and vitamins. Many nonprescription products contain drugs which may interact with guanethidine (e.g., diet pills and medications for colds and asthma), so check labels carefully. Do not take any of these medications without talking to your doctor (even if you never had a problem taking them before).
  • tell your doctor if you have or have ever had asthma, ulcers, congestive heart failure, coronary artery disease, kidney disease, pheochromocytoma, or a heart attack.
  • tell your doctor if you are pregnant, plan to become pregnant, or are breast-feeding. If you become pregnant while taking guanethidine, call your doctor.
  • if you are having surgery, including dental surgery, tell the doctor or dentist that you are taking guanethidine. You may be told to stop taking guanethidine 2 weeks prior to surgery; follow your doctor's directions.
  • Ask your doctor about the safe use of alcohol while you are using guanethidine. Alcohol can make the side effects from guanethidine worse.

Special dietary instructions

Your doctor may prescribe a low-salt or low-sodium diet. Follow these directions carefully.

What to do if you forget a dose

Take the missed dose as soon as you remember it. However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Do not take a double dose to make up for a missed one.

Side effects

Minor side effects

Although side effects from guanethidine are not common, they can occur. Tell your doctor if any of these symptoms are severe or do not go away:

  • drowsiness
  • dizziness
  • tiredness
  • headache
  • confusion
  • decreased sexual ability
  • upset stomach
  • loss of appetite
  • gas
  • constipation
  • vomiting
  • dry mouth
  • frequent urination
  • muscle pain
  • depression
  • difficulty sleeping

Severe side effects

If you experience any of the following symptoms, call your doctor immediately:

  • severe diarrhea
  • fainting
  • fever
  • increased or decreased heartbeat
  • irregular heartbeat
  • chest pain
  • swollen ankles or feet
  • shortness of breath

If you experience a serious side effect, you or your doctor may send a report to the Food and Drug Administration's (FDA) MedWatch Adverse Event Reporting program online [at] or by phone [1-800-332-1088].

Storage conditions needed for this medication

Keep this medication in the container it came in, tightly closed, and out of reach of children. Store at room temperature and away from excess heat and moisture (not in the bathroom). Throw away any medication that is outdated or no longer needed. Talk to your pharmacist about the proper disposal of your medication.

In case of emergency/overdose

In case of overdose, call your local poison control center at 1-800-222-1222. If the victim has collapsed or is not breathing, call local emergency services at 911.

Other information

Keep all appointments with your doctor and the laboratory. Your blood pressure should be checked regularly to determine your response to guanethidine.

To relieve dry mouth caused by guanethidine, chew gum or suck sugarless hard candy.

Do not let anyone else take your medication. Ask your pharmacist any questions you have about refilling your prescription.

Brand names

  • Ismelin®

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