I have taken both Corgard and Namenda, Ashley, and I can tell you that med's have different side effects in different people. Corgard has a slight % of people who will gain weight, however I didn't when I took this (which is really saying something). Namenda is interesting; it's actually used for Alzheimers, but off label for Migraine. Weight gain is not an issue with this drug.
From my experience, and with the frequency and level of pain that I have when I go into a cluster period, is that food is comfort; it relieves...even for a short time...pain, and that, in combination with the meds equals weight. Saying that, some drugs (depakote for one) can cause tremendous weight gain in a short period of time. Keep a diary, including your weight and your appetite; you may find a correlation. Watch your calories and increase your exercise if you can, it may stave off the extra pounds. If it becomes a problem, talk to your Dr. about changing your med. Unfortunately, (and I think quite unfairly!) the weight doesn't come off when you stop taking the med.