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Ismelin Side Effects

Generic Name: guanethidine

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

For the Consumer

Applies to guanethidine: oral tablet

Along with its needed effects, guanethidine (the active ingredient contained in Ismelin) 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 while taking guanethidine:

More Common

  • Swelling of feet or lower legs

Less Common or Rare

  • Chest pain
  • shortness of breath

Some side effects of guanethidine 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

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 The manufacturer recommends that guanethidine (the active ingredient contained in Ismelin) be gradually withdrawn over at least two weeks prior to administration of general anesthetics to avoid cardiovascular collapse during induction.[Ref]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]


1. Walter IE, Nies AS "Safety of single large oral doses of guanethidine." Clin Pharmacol Ther 21 (1977): 706-8

2. Hansson L, Pascual A, Julius S "Comparison of guanadrel and guanethidine." Clin Pharmacol Ther 14 (1973): 204-8

3. Sharpe E, Milaszkiewicz R, Carli F "A case of prolonged hypotension following intravenous guanethidine block." Anaesthesia 42 (1987): 1081-4

4. Kalmanovitch DV, Hardwick PB "Hypotension after guanethidine block." Anaesthesia 43 (1988): 256

5. Grunstein JA "The problem of postural hypotension." Gerontol Clin (Basel) 16 (1974): 171-4

6. Leishman AW, Sandler G "Guanethidine and hypertension after five years." Angiology 18 (1967): 705-16

7. 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 10 (1995): 13-20

8. Kaplan R, Claudio M, Kepes E, Gu XF "Intravenous guanethidine in patients with reflex sympathetic dystrophy." Acta Anaesthesiol Scand 40 (1996): 1216-22

9. Dollery CT, Emslie-Smith D, Milne MD "Clinical and pharmacological studies with guanethidine in the treatment of hypertension." Lancet Aug (1960): 381-7

10. Adi FC, Eze CJ, Anwunah A "Comparison of debrisoquine and guanethidine in treatment of hypertension." Br Med J Mar (1975): 482-5

11. Scheinman MM, Strauss HC, Evans GT, et al "Adverse effects of sympatholytic agents in patients with hypertension and sinus node dysfunction." Am J Med 64 (1978): 1013-20

12. "Product Information. Ismelin (guanethidine)." Ciba Pharmaceuticals, Summit, NJ.

13. Malinow SH "Comparison of guanadrel and guanethidine efficacy and side effects." Clin Ther 5 (1983): 284-9

14. Ramirez EA, Elson L, Freis ED, et al "Multiclinic controlled trial of bethanidine and guanethidine in severe hypertension." Circulation 55 (1977): 519-25

15. Goldberg LI, Zimmerman AM "Guanethidine and methyldopa as therapeutic agents in hypertension: a comparative review." Postgrad Med June (1963): 548-54

16. Woosley RL, Nies AS "Guanethidine." N Engl J Med 295 (1976): 1053-8

17. Bulpitt CJ, Dollery CT "Side effects of hypotensive agents evaluated by a self-administered questionnaire." Br Med J 3 (1973): 485-90

18. Weil JV, Chidsey CA "Plasma volume expansion resulting from interference with adrenergic function in normal man." Circulation 37 (1968): 54-61

19. Tarpley EL "Controlled trial of guanethidine and methyldopa in moderate hypertension." Curr Ther Res Clin Exp 16 (1974): 1187-96

20. Rowe J, Bassan MM "Symptomatic sick sinus syndrome due to guanethidine." Hypertension 1 (1979): 543-6

21. Bauer GE, Hull RD, Stokes GS, Raftos J "The reversibility of side effects of guanethidine therapy." Med J Aust May (1973): 930-3

22. Kedia K, Markland C "The effect of pharmacological agents on ejaculation." J Urol 114 (1975): 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.