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Guanadrel Side Effects

Medically reviewed by Last updated on Nov 22, 2022.

Applies to guanadrel: oral tablet.

Nervous system

The most common problems associated with guanadrel affect the nervous system. Up to 60% of patients have experienced drowsiness, fatigue, or headaches, 25% of patients experienced paresthesias or visual disturbances, and 2% experienced sleep disorders during therapy[Ref]


Cardiovascular side effects can result from excessive sympathetic blockade or a relative increase in parasympathetic tone. Orthostatic hypotension occurs in 8% to 50% 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.

Peripheral edema has been reported in up to 30% of patients.

Other cardiovascular problems include chest pain in 28%, dyspnea at rest in 18%, dyspnea on exertion in 46%, and palpitations in 30% of patients. Underlying diseases may account for the relatively high incidence of some of these side effects.[Ref]

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 The manufacturer recommends that guanadrel be gradually withdrawn over at least two weeks prior to administration of general anesthetics to avoid cardiovascular collapse during induction.[Ref]


Gastrointestinal side effects are also related to increased parasympathetic tone. Diarrhea has been reported in 5% to 30% of patients, some of whom discontinue therapy because of it. Constipation has been reported in up to 21% of patients. Dry mouth or parotid tenderness have been associated with the use of this drug in approximately 2% of patients.[Ref]


There is evidence that peripherally-acting antiadrenergic drugs may interfere with ejaculation by inhibiting contraction of the seminal vesicle, ampula and ductus deferens.[Ref]

Sexual impotence is a relatively common genitourinary complaint, occurring in 5% to 18% of male patients. Smaller studies, where specific questions were asked, revealed an incidence of impotence as high as 60% of male patients who were receiving a similar drug, guanethidine. Impotence appears to be reversible upon discontinuation of therapy or reduction in dosage. Urinary frequency or urgency has occurred in 30% to 50% of patients.[Ref]


Musculoskeletal cramping has occurred in approximately 20% to 40% of patients.[Ref]


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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.