Sodium and water retention: Frequently occurred in patients receiving minoxidil and may result in edema, weight gain, congestive heart failure, pulmonary edema, and "refractoriness" to the antihypertensive effects of the drug. Concomitant administration of a diuretic is required except in some patients who are undergoing hemodialysis. Diuretic therapy, alone or with salt restriction, usually minimizes fluid retention, although reversible edema did develop in approximately 10% of patients who were treated in this manner and were not undergoing dialysis. Ascites also has been reported. Rarely, refractory fluid retention may occur, requiring discontinuance of minoxidil; in some patients who can be closely supervised, refractory fluid retention may be treated by discontinuing minoxidil for 1-2 days and then resuming minoxidil therapy in conjunction with vigorous diuretic therapy. In patients on hemodialysis, fluid retention can be controlled with more vigorous ultrafiltration.
ECG changes: ECG changes in the magnitude and direction of the T waves (i.e., positive T waves flatten or invert and negative T waves show increased negativity) occur commonly. Rarely, large negative amplitude of the T wave may encroach upon the ST segment, but the ST segment alone is not changed. ECG changes usually revert to the pretreatment state with continued therapy or when minoxidil is discontinued. No symptoms, evidence of myocardial damage, or deterioration of cardiac function have been associated with minoxidil-induced ECG changes.
Tachycardia/Angina: Minoxidil increase heart rate; this can be minimized by concomitant administration of a β-adrenergic blocking agent or other sympathetic nervous system suppressant. Angina pectoris may worsen or occur in patients without previous angina, probably due to increased oxygen demand associated with increased heart rate and cardiac output and can usually be prevented by a β-blocker or other sympathetic nervous system suppressant.
Pericardial effusion: Occasionally with tamponade, it has occurred in about 3% of patients receiving minoxidil who were not on dialysis, especially in those with inadequate or compromised renal function.
Although pericardial effusion has occurred most often in patients with a connective tissue disease, uremic syndrome, congestive heart failure, or marked fluid retention, there have been instances in which these potential causes of effusion were not present. Patients receiving minoxidil should be observed for signs and symptoms of pericarditis, pericardial effusion, and tamponade, and echocardiograms should be performed if necessary. More vigorous diuretic therapy, dialysis, pericardiocentesis, or surgery may be required and, if effusion persists, withdrawal of minoxidil should be considered. Pericardial effusion is thought to result from minoxidil-induced sodium and water retention. Pericarditis also has been reported in minoxidil-treated patients; however, the relationship of this effect to renal function is unclear.
Hypertrichosis: Within 3-6 weeks after initiating minoxidil therapy, hypertrichosis (elongation, thickening, and increased pigmentation of fine body hair) commonly occurs but is not associated with an endocrine abnormality. Occasionally hypertrichosis may be associated with apparent coarsening of facial features, which may be due to mild generalized fluid retention; transient pruritus may also be associated with hair growth.
Hemodilution: Hematocrit, hemoglobin concentration, and erythrocyte count usually decrease about 7% and then return to pretreatment levels.