Memory problems can be exacerbated by the medication, but it is actually a problem of bipolar disorder itself. Cognitive function (not intelligence) actually suffers a great deal with bipolar. Some people are worse than others (just like all symptoms are worse in some than others.)

Anyway, there's a ton of research going on with cognitive dysfunction and bipolar. I actually have many cognitive functioning problems. Memory, processing, organization, and focus are the primary ones I deal with.

For my med history, I've barely ever been on meds. I was diagnosed in 2009 and have maybe had 9 months of meds total in all this time, but my cognitive functioning has been in continuous decline. I just restarted lithium last week.