Guanethidine adverse effects

B) Guanethidine causes fewer CNS adverse effects (such as sedation) than methyldopa... 

C) Guanethidine causes more immunologic adverse effects (eg, hemolytic anemia) than methyldopa... 

Guanethidine causes many peripheral adverse effects but is poorly distributed into the CNS, so it is relatively free of CNS effects. The answer is (B). 

You are considering therapeutic options for a new patient who presents with severe hypertension and angina. In considering adverse effects, you note that an adverse effect which nitroglycerin, guanethidine, and ganglion blockers have in common is (A) Bradycardia... 

Overall, the frequency of adverse effects produced by guanadrel is similar or less than those produced by guanethidine and by methyidopa. In patients with impaired renal function, the elimination half-life of unmetabolized guanadrel is prolonged and its clearance decreased, thus increasing the incidence of adverse effects if the usual dosage is maintained in these patients. 

A brief report describes a patient taking guanethidine and a diuretic who, when given levodopa (dose not stated), required a reduction in his daily dose of guanethidine, from 60 to 20 mg. Another patient similarly treated was able to discontinue the diuretic. The suggested reason is that the hypotensive adverse effects of the levodopa are additive with the effects of the guanethidine. Direct information seems to be limited to this report but it would be wise to confirm that excessive hypotension does not develop if levodopa is added to treatment with guanethidine. 

Patients taking certain systemic medications are also more sensitive to the pressor effects of phenylephrine. In individuals taking atropine, the pressor effect of phenylephrine is augmented, and tachycardia can occur. Tricyclic antidepressants and monoamine oxidase (MAO) inhibitors also potentiate the cardiovascular effects of topical phenylephrine. The concomitant use of phenylephrine is contraindicated with these agents, even up to 21 days after cessation of MAO inhibitor therapy. Similarly, patients taking reserpine, guanethidine, or methyldopa are at increased risk for adverse pressor effects from topical phenylephrine because of denervation hypersensitivity accompanying the chemical sympathectomy. 

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