The more frequent early adverse effects (usually transitory) are: tachycardia, headache, palpitation, dizziness, weakness, nausea, vomiting, and postural hypotension. Less frequently numbness and tingling of the extremities, flushing, depression, nasal congestion, lacrimation, conjuctival injection, edema, tremors, dyspnea, skin rash, drug fever, muscle cramps, anginal symptoms, and giant urticaria. Adverse effects tend to disappear as treatment is continued. When they do not remit spontaneously or upon reduction of dosage, combination therapy with other agents such as reserpine or hydrochlorothiazide may be tried.
Peripheral neuritis, evidenced by paresthesias, numbness and tingling has been observed. Published evidence suggests an antipyridoxine effect and addition of pyridoxine to the regimen if symptoms develop.
Blood dyscrasias, consisting of reduction in hemoglobin and red cell count, leukopenia, agranulocytosis and purpura have been reported rarely. If such abnormalities develop, discontinue therapy.
Late adverse effects: An arthritis-like syndrome has been observed in few a cases, usually after hydralazine was administered for long periods and in relatively high doses. The milder phase of the syndrome is characterized by migratory arthralgia in the joints of the hands and less commonly in the wrist, elbows, shoulders and knees. It may or may not be accompanied by fever. Continued use of hydralazine in these cases has been reported to lead to a clinical picture simulating acute systemic lupus erythematosus.
When early arthritis-like symptoms appear during therapy with hydralazine, spontaneous remission of symptoms usually follows withdrawal of the drug. The late reactions, which develop when treatment is continued despite the appearance of early symptoms, are more severe and longer lasting than the early phase. Most of these reactions have been completely reversible. Some patients may require protracted therapy, including ACTH and cortisone.