Encainide hydrochloride is primarily used as an antiarrhythmic agent to treat certain types of irregular heartbeats, specifically ventricular arrhythmias. It works by blocking sodium channels in the heart, which helps to stabilize the heart's electrical activity and restore a normal rhythm. This makes it effective in managing life-threatening arrhythmias that could lead to more severe cardiac events. However, its use has declined over time due to concerns about proarrhythmic effects and the availability of safer alternatives. It is typically prescribed under strict medical supervision, often in hospital settings, to monitor its efficacy and potential side effects.