Multaq is among the least 'offensive' of the commonly prescribed anti-arrhythmic drugs, but it also means it may not work terrible well or long. In fact, most people with paroxysmal AF, which is a progressive disorder (not that I'm assuming that AF is the arrhythmia you have, it could be another) tend to need more of the original prescription in time, or they need to switch to another drug that they can both tolerate well and that does the trick.

Your prescriber may have selected it just to see if it works, or he/she may know something about you, your genetics and other medical circumstances, that make other choices undesirable...for now. As you found, your genetics don't allow you to tolerate metoprolol well, but most people do just fine on it...including me.

Generally, all anti-arrhythmic drugs are safe, barring a genetic prohibition, for the vast majority of patients...which is why they are so routinely prescribed. However, every patient has the responsibility to apprise their caregivers of adverse reactions, and quickly. While nobody likes being on any medication, especially for life (Eliquis and Plavix, for example, if your CHA2Ds2-VASc score is upwards of 1), I think most would agree that it is at least somewhat better than having your racing heart keep you awake at night, or running the risk of a stroke, or of heart insufficiency.