What Are The Side Effects Of Calaptin?

I am a 21 year boy.suffered from supra ventricular tachycardia (SVT) at the age of 17.have been taking calaptin 40 mg two tablets in a day since then.no signs of the problem have ever surfaced back.can I leave the medicine now.what are the dangers associated if I take the medicine intermittently like I have been doing for last 6-8 months.does this render medicine ineffective or have side effects.