In the U.S.—

In Canada—

• Antihypertensive

Guanethidine (gwahn-ETH-i-deen) belongs to the general class of medicines called antihypertensives. It is used to treat high blood pressure (hypertension).

High blood pressure adds to the work load of the heart and arteries. If it continues for a long time, the heart and arteries may not function properly. This can damage the blood vessels of the brain, heart, and kidneys, resulting in a stroke, heart failure, or kidney failure. High blood pressure may also increase the risk of heart attacks. These problems may be less likely to occur if blood pressure is controlled.

Guanethidine works by controlling nerve impulses along certain nerve pathways. As a result, it relaxes the blood vessels so that blood passes through them more easily. This helps to lower blood pressure.

Guanethidine is available only with your doctor's prescription, in the following dosage form:

Oral
• Tablets (U.S. and Canada)

In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For guanethidine, the following should be considered:

Allergies—Tell your doctor if you have ever had any unusual or allergic reaction to guanethidine. Also tell your health care professional if you are allergic to any other substances, such as foods, preservatives, or dyes.

Pregnancy—Studies on effects in pregnancy have not been done in either humans or animals.

Breast-feeding—Small amounts of guanethidine pass into breast milk. However, this medicine has not been reported to cause problems in nursing babies.

Children—Although there is no specific information comparing use of guanethidine in children with use in other age groups, this medicine is not expected to cause different side effects or problems in children than it does in adults.

Older adults—Many medicines have not been studied specifically in older people. Therefore, it may not be known whether they work exactly the same way they do in younger adults. Although there is no specific information comparing use of guanethidine in the elderly with use in other age groups, dizziness, lightheadedness, or fainting may be more likely to occur in the elderly, who are more sensitive to the effects of guanethidine.

Other medicines—Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking guanethidine, it is especially important that your health care professional knows if you are taking any of the following:

• Antidiabetics, oral (diabetes medicine you take by mouth)—Effects may be increased by guanethidine

• Loxapine (e.g., Loxitane) or
• Thioxanthenes (chlorprothixene [e.g., Taractan], thiothixene [e.g., Navane]) or
• Tricyclic antidepressants (amitriptyline [e.g., Elavil], amoxapine [e.g., Asendin], clomipramine [e.g., Anafranil], desipramine [e.g., Pertofrane], doxepin [e.g., Sinequan], imipramine [e.g., Tofranil], nortriptyline [e.g., Aventyl], protriptyline [e.g., Vivactil], trimipramine [e.g., Surmontil]) or
• Trimeprazine (e.g., Temaril)—May decrease the effects of guanethidine on blood pressure

• Minoxidil (e.g., Loniten)—Effects on blood pressure may be greatly increased

• Monoamine oxidase (MAO) inhibitor activity (isocarboxazid [e.g., Marplan], phenelzine [e.g., Nardil], procarbazine [e.g., Matulane], selegiline [e.g., Eldepryl], tranylcypromine [e.g., Parnate])—Taking guanethidine while you are taking or within 1 week of taking MAO inhibitors may cause a severe increase in blood pressure

• Asthma (history of) or
• Diarrhea or
• Pheochromocytoma or
• Stomach ulcer (history of)—Guanethidine may worsen these conditions

• Diabetes mellitus (sugar diabetes)—Effects of medicine used to treat this condition may be increased by guanethidine

• Fever—Effects of guanethidine may be increased

• Heart attack or stroke (recent) or
• Heart or blood vessel disease—Lowering blood pressure may make problems resulting from these conditions worse

• Kidney disease—Guanethidine may worsen this condition. Also, effects of guanethidine may be increased because of slower removal of this medicine from the body

• Liver disease—Effects of guanethidine may be increased because of slower removal from the body

In addition to the use of the medicine your doctor has prescribed, treatment for your high blood pressure may include weight control and care in the types of foods you eat, especially foods high in sodium. Your doctor will tell you which of these are most important for you. You should check with your doctor before changing your diet.

Many patients who have high blood pressure will not notice any signs of the problem. In fact, many may feel normal. It is very important that you take your medicine exactly as directed and that you keep your appointments with your doctor even if you feel well.

Remember that guanethidine will not cure your high blood pressure but it does help control it. Therefore, you must continue to take it as directed if you expect to lower your blood pressure and keep it down. You may have to take high blood pressure medicine for the rest of your life . If high blood pressure is not treated, it can cause serious problems such as heart failure, blood vessel disease, stroke, or kidney disease.

To help you remember to take your medicine, try to get into the habit of taking it at the same time each day.

Dosing—

The dose of guanethidine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of guanethidine. If your dose is different, do not change it unless your doctor tells you to do so.

The number of tablets that you take depends on the strength of the medicine.

• For oral dosage form (tablets):
  • For high blood pressure:
    • Adults—At first, 10 or 12.5 milligrams (mg) once a day. Then, your doctor may increase your dose to 25 to 50 mg once a day.
    • Children—The dose is based on body weight and must be determined by your doctor. The usual dose is 200 micrograms (mcg) per kilogram (kg) (90.9 mcg per pound) of body weight a day. Then, your doctor may increase your dose as needed.

Missed dose—

If you miss a dose of guanethidine, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not double doses.

Storage—

To store this medicine:

• Keep out of the reach of children.
• Store away from heat and direct light.
• Do not store in the bathroom, near the kitchen sink, or in other damp places. Heat or moisture may cause the medicine to break down.
• Do not keep outdated medicine or medicine no longer needed. Be sure that any discarded medicine is out of the reach of children.

It is important that your doctor check your progress at regular visits to make sure that this medicine is working properly.

Dizziness, lightheadedness, or fainting may occur, especially when you get up from a lying or sitting position. This is more likely to occur in the morning. Getting up slowly may help. When you get up from lying down, sit on the edge of the bed with your feet dangling for 1 or 2 minutes. Then stand up slowly. If the problem continues or gets worse, check with your doctor.

The dizziness, lightheadedness, or fainting is also more likely to occur if you drink alcohol, stand for long periods of time, exercise, or if the weather is hot. While you are taking this medicine, be careful in the amount of alcohol you drink. Also, use extra care during exercise or hot weather or if you must stand for long periods of time.

Do not take other medicines unless they have been discussed with your doctor. This especially includes over-the-counter (nonprescription) medicines for appetite control, asthma, colds, cough, hay fever, or sinus problems, since they may tend to increase your blood pressure.

Before having any kind of surgery (including dental surgery) or emergency treatment, tell the medical doctor or dentist in charge that you are taking this medicine.

Tell your doctor if you get a fever since that may change the amount of medicine you have to take.

Male patients: This medicine may interfere with ejaculation.

Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• More common
  • Swelling of feet or lower legs 
  
  

• Less common or rare
  • Chest pain;  shortness of breath 
  
  

Other side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. However, check with your doctor if any of the following side effects continue or are bothersome:

• More common
  • Diarrhea or increase in bowel movements;  dizziness, lightheadedness, or fainting, especially when getting up from a lying or sitting position;  sexual problems in males ;  slow heartbeat;  stuffy nose;  unusual tiredness or weakness 
  
  

• Less common or rare
  • Blurred vision;  drooping eyelids;  dryness of mouth;  headache;  loss of hair on scalp;  muscle pain or tremors;  nausea or vomiting;  nighttime urination;  skin rash 
  
  

Other side effects not listed above may also occur in some patients. If you notice any other effects, check with your doctor.