Pronunciation:

gwahn-ETH-i-deen

Brand Names:

• Ismelin

Classifications:

Pharmacologic—

Adrenergic Blocker

Uses of This Medicine:

Guanethidine 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.

Before Using This Medicine:

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 this medicine, the following should be considered:

Allergies—

Tell your doctor if you have ever had any unusual or allergic reaction to this medicine or any other medicines. Also tell your health care professional if you have any other types of allergies, such as to foods, dyes, preservatives, or animals. For non-prescription products, read the label or package ingredients carefully.

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.

Breast-feeding—

There are no adequate studies in women for determining infant risk when using this medication during breastfeeding. Weigh the potential benefits against the potential risks before taking this medication while breastfeeding.

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 this medicine, it is especially important that your healthcare professional know if you are taking any of the medicines listed below. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.

Using this medicine with any of the following medicines is not recommended. Your doctor may decide not to treat you with this medication or change some of the other medicines you take.

• Brofaromine
• Clorgyline
• Furazolidone
• Iproniazid
• Isocarboxazid
• Lazabemide
• Linezolid
• Moclobemide
• Nialamide
• Pargyline
• Phenelzine
• Procarbazine
• Rasagiline
• Selegiline
• Toloxatone
• Tranylcypromine

Using this medicine with any of the following medicines is usually not recommended, but may be required in some cases. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.

• Iobenguane I 123
• Phenylpropanolamine
• Pseudoephedrine

Using this medicine with any of the following medicines may cause an increased risk of certain side effects, but using both drugs may be the best treatment for you. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.

• Amitriptyline
• Doxepin
• Phenylephrine
• Semaglutide
• Yohimbine

Other interactions—

Certain medicines should not be used at or around the time of eating food or eating certain types of food since interactions may occur. Using alcohol or tobacco with certain medicines may also cause interactions to occur. Discuss with your healthcare professional the use of your medicine with food, alcohol, or tobacco.

Other medical problems—

The presence of other medical problems may affect the use of this medicine. Make sure you tell your doctor if you have any other medical problems, especially:

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

• Type 2 diabetes mellitus—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

Proper Use of This Medicine:

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 this medicine 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 this medicine. If your dose is different, do not change it unless your doctor tells you to do so.

The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using 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 this medicine, 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—

Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light. Keep from freezing.

Keep out of the reach of children.

Do not keep outdated medicine or medicine no longer needed.

Precautions While Using This Medicine:

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.

Side Effects of This Medicine:

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

Some 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. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

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 may also occur in some patients. If you notice any other effects, check with your healthcare professional.

Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.