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Ismelin Side Effects

Generic name: guanethidine

Medically reviewed by Last updated on Mar 4, 2023.

Note: This document contains side effect information about guanethidine. Some dosage forms listed on this page may not apply to the brand name Ismelin.

Applies to guanethidine: oral tablet.


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.

If you experience any of the following serious side effects, stop taking guanethidine (the active ingredient contained in Ismelin) 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.

For Healthcare Professionals

Applies to guanethidine: oral tablet.


The risk of orthostatic hypotension, sometimes followed by syncope, is greatest within the first 10 minutes after dosing, early in the morning, and in hypovolemic patients. It is accentuated by alcohol, hot weather, or exercise--all of which are associated with peripheral vasodilation. The manufacturer recommends that guanethidine (the active ingredient contained in Ismelin) be gradually withdrawn over at least two weeks prior to administration of general anesthetics to avoid cardiovascular collapse during induction.[Ref]

Cardiovascular side effects can result from excessive sympathetic blockade or a relative increase in parasympathetic tone. Orthostatic hypotension is reported in approximately 15% of patients, some of whom experience syncope.

Unopposed or excessive parasympathetic tone can cause excessive bradycardia in rare cases. This may cause serious problems in patients with underlying sinus node dysfunction.

Guanethidine can cause generalized edema in 10% to 15% of patients. Some patients with preexisting congestive heart failure do not tolerate this, and require concomitant diuretic therapy.[Ref]


Gastrointestinal side effects are also related to increased parasympathetic tone. Diarrhea is reported in 11% of patients, some of whom discontinued therapy because of it. Dry mouth or parotid tenderness are reported in approximately 5% of patients.[Ref]


There is evidence that guanethidine (the active ingredient contained in Ismelin) may interfere with ejaculation by inhibiting contraction of the seminal vesicle, ampula and ductus deferens.[Ref]

Large studies report sexual impotence as a relatively uncommon genitourinary complaint, occurring in only approximately 2% of male patients. Smaller studies, where specific questions were asked of male patients reveal an incidence of impotence as high as 60%. Delayed or retrograde ejaculation and decreased sperm counts are reported. Impotence appears to be reversible upon discontinuation of therapy or reduction in dosage.[Ref]


A relatively common respiratory system complaint, nasal stuffiness in up to 30% of patients, is related to increased parasympathetic tone. Rare reports of dyspnea or exertion unaccompanied by other signs or symptoms of congestive heart failure are associated with guanethidine (the active ingredient contained in Ismelin) [Ref]


In one study, 58% of patients with or without preexisting elevated BUN developed an increase in the BUN during guanethidine (the active ingredient contained in Ismelin) therapy. The study did not, however, quantify the rise in BUN, attempt to make an association with the degree of blood pressure control, or attempt to measure other parameters of renal function.[Ref]

Nervous system

Because guanethidine (the active ingredient contained in Ismelin) does not affect the central nervous system, neurologic side effects are notably either absent or infrequent. Insomnia and weakness are occasionally reported more often with guanethidine than with placebo.[Ref]


Hypersensitivity reactions are not associated with guanethidine (the active ingredient contained in Ismelin) [Ref]


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18. Product Information. Ismelin (guanethidine). Ciba-Geigy Pharmaceuticals. 2001.

19. Jadad AR, Carroll D, Glynn CJ, Mcquay HJ. Intravenous regional sympathetic blockade for pain relief in reflex sympathetic dystrophy: a systematic review and a randomized, double-blind crossover study. J Pain Symptom Manage. 1995;10:13-20.

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22. Kedia K, Markland C. The effect of pharmacological agents on ejaculation. J Urol. 1975;114:569-73.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Some side effects may not be reported. You may report them to the FDA.