Flecainide was initially prescribed at a dose of 200 mg twice daily, but after early toxicity in patients with ventricular tachycardia (VT), the dosage was reduced to 100 mg twice daily. The effects of flecainide were studied in 40 patients (29 men and 11 women, mean age 62 +/- 2 years, ejection fraction 45 +/- 3%) who underwent programmed electrical stimulation to determine the efficacy of flecainide in preventing VT chronically at the reduced dose. Sustained VT was induced in 21 patients and nonsustained VT in 19. Flecainide prevented VT induction in 26 patients (65%). At a mean dose of 1.5 +/- 0.1 mg/kg, prolongation occurred in the effective refractory period of the first (280 +/- 5 vs 249 +/- 5 ms) and second (254 +/- 6 vs 209 +/- 9 ms) extrastimuli (p less than 0.01). In the patients protected by flecainide, the effective refractory periods increased by a 17 +/- 2% and 21 +/- 3%, in contrast to only a 7 +/- 3% and 6 +/- 4% increase in the nonprotected group (p less than 0.05), despite a higher mean dose (1.9 +/- 0.1 vs 1.35 +/- 0.1 mg/kg). Twenty-one patients were discharged on flecainide therapy, 100 mg twice daily, and were followed for a mean of 11 months. Sixteen patients are alive and well, 1 died suddenly, 1 died from a noncardiac cause and 1 had a "breakthrough" arrhythmia. Two were switched to quinidine therapy by their referring physicians, but were without problems while receiving flecainide.(ABSTRACT TRUNCATED AT 250 WORDS)