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Discontinued Warning IconPlease Note: This Brand Name drug is no longer available in the US.
(Generic versions may still be available.)



Ismelin (guanethidine monosulfate) is a potent drug and its use can lead to disturbing and serious clinical problems. Before prescribing, physicians should familiarize themselves with the details of its use and warn patients not to deviate from instructions.

Orthostatic hypotension can occur frequently, and patients should be properly instructed about this potential hazard. Fainting spells may occur unless the patient is forewarned to sit or lie down with the onset of dizziness or weakness. Postural hypotension is most marked in the morning and is accentuated by hot weather, alcohol, or exercise. Dizziness or weakness may be particularly bothersome during the initial period of dosage adjustment and with postural changes, such as arising in the morning. The potential occurrence of these symptoms may require alteration of previous daily activity. The patient should be cautioned to avoid sudden or prolonged standing or exercise while taking the drug.

Inhibition of ejaculation has been reported in animals (see


: Carcinogenesis, Mutagenesis Impairment of Fertility) as well as in men given Ismelin (guanethidine monosulfate) . This effect, which results from the sympathetic blockade caused, by the drug's action, is reversible after Ismelin (guanethidine monosulfate) has been discontinued for several weeks. The drug does not cause parasympathetic blockade, and erectile potency is, usually retained during administration of Ismelin (guanethidine monosulfate) . The possible occurrence of inhibition of ejaculation should be kept in mind when considering the use of guanethidine in men of reproductive age.

If possible, therapy should be withdrawn 2 weeks prior to surgery to reduce the possibility of vascular collapse and cardiac arrest during anesthesia. If emergency surgery is indicated, preanesthetic and anesthetic agents should be administered cautiously in reduced dosage. Oxygen, atropine, vasopressors, and adequate solutions for volume replacement should be ready for immediate use to counteract vascular collapse in the surgical patient. Vasopressors should be used only with extreme caution, since Ismelin (guanethidine monosulfate) augments responsiveness to exogenously administered norepinephrine and vasopressors; specifically, blood pressure may rise and cardiac arrhythmias may be produced.



Dosage requirements may be reduced in the presence of fever.

Special care should be exercised when treating patients with a history of bronchial asthma; asthmatic patients are more apt to be hypersensitive to catecholamine depletion, and their condition may be aggravated.

The effects of Ismelin (guanethidine monosulfate) are cumulative over long periods; initial doses should be small and increased gradually in small increments.

Ismelin (guanethidine monosulfate) should be used very cautiously in hypertensive patients with: renal disease and nitrogen retention or rising BUN levels, since decreased blood pressure may further compromise renal function; coronary insufficiency or recent myocardial infarction, and cerebrovascular disease, especially with encephalopathy.

Ismelin (guanethidine monosulfate) should not be given to patients with severe cardiac failure except with extreme caution since Ismelin (guanethidine monosulfate) may interfere with the compensatory role of the adrenergic system in producing circulatory adjustment in patients with congestive heart failure.

Patients with incipient cardiac decompensation should be watched for weight gain or edema, which may be averted by the concomitant administration of a thiazide.

Ismelin (guanethidine monosulfate) should be used cautiously in patients with a history of peptic ulcer or other chronic disorders that may be aggravated by a relative increase in parasympathetic tone.

Information for Patients


Drug Interactions


Carcinogenesis, Mutagenesis, Impairment of Fertility

Long-term carcinogenicity studies in animals have not been conducted with Ismelin (guanethidine monosulfate) .

While inhibition of sperm passage and accumulation of sperm debris have been reported in rats and rabbits after several weeks of administration of Ismelin (guanethidine monosulfate) , 5 or 10 mg/kg per day, subcutaneously or intraperitoneally, recovery of ejaculatory function and fertility has been demonstrated in rats given Ismelin (guanethidine monosulfate) intramuscularly, 25 mg/kg per day, for 8 weeks. Inhibition of ejaculation has also been reported in men (see


and ADVERSE REACTIONS). This effect, which is attributable to the sympathetic blockade caused by the drug, is reversible several weeks after discontinuance of the drug.

Pregnancy Category C

Animal reproduction studies have not been conducted with Ismelin (guanethidine monosulfate) . It is also not known whether Ismelin (guanethidine monosulfate) can cause fetal harm when administered to a pregnant woman or can affect reproduction capacity. Ismelin (guanethidine monosulfate) should be given to a pregnant woman only if clearly needed.

Nursing Mothers

Ismelin (guanethidine monosulfate) is excreted in breast milk in very small quantity. Caution should be exercised when Ismelin (guanethidine monosulfate) is administered to a nursing woman.

Pediatric Use

Safety and effectiveness in pediatric patients have not been established.

This monograph has been modified to include the generic and brand name in many instances.

Last reviewed on RxList: 12/8/2004


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