I have been on rhythmol before (2 years), but my doctor made me get off it. It is a powerful and sometimes dangerous anti arrhythmic. Usually patients are hospitalized when it is initially administrated because it can cause dangerous heart arrhythmias. I was hospitalized for 3 days when I first started taking it. You have to be monitored closely. I am now on Toprol XL - a very small dose, and it makes my pvcs lessen and even go away. It is a pretty safe drug and you must try and take the smallest dose possible. Write again and let me know how you are doing.
Here's a slightly different perspective. I have tried Rythmol SR at a high dose level of 425mg twice a day. I was at "home" on a monitor when I started this medication because of its potential for problems... but I had not, it also did nothing to stop my Atrial Fibrillation. The next step up according to my doctor is Flecanide and he would not put me on that medication unless I agreed to check in to the hospital and be put on full-time monitors for a few days to be sure I didn't have an life threatening episodes due to the drug. I decided not to take Flecanide.
I am now on what is called "rate control" meaning I'm living with AFib and use a beta blocker, Metoprolol, to keep my ventricle HR at a reasonable rate, in the 80s at rest for me.