I suffer nasty side effects when I take medicine for congestion. If you suffer similarly - I KNOW I'm not alone on this - and you want to take such medicines anyway (I certainly do), try this: Greatly Lower the Dosage. For example, if you're supposed to take two pills every four hours, take one every eight. (This doesn't work very well with 24-hour pills, I'm afraid.)
The theory is simple: If the side effects affect you much more strongly than they're supposed to, maybe the same holds true for the intended effects.
Obligatory disclaimer: IANAD, IANAPh, IANAH, IANAA, in fact, IANAx, where 'x' might be almost anything.