I've been wondering this lately, too.
I went on Lopressor in January (for atrial fibrillation and high blood pressure) and it keeps my heart rate very low. The first 2 months I was on it I was exhausted all the time. Now I'm tired, but not exhausted. I've gained 10+ lbs since January, at least due to my decreased activity level.
I joined WW this month, and started taking walks two weeks ago, gradually increasing the minutes, but it's very tiring. If during aerobic exercise you need to get your heart rate up to a certain level in order to lose weight, but the medication keeps you from achieving that level, surely that will affect your weight loss?
I've lost 8 lbs so far, though, so it won't keep me from trying! Besides, I'm sure my heart likes the good food and exercise
The last time I saw my doctor she said she might adjust my Lopressor, or decrease it and add something else, in order to bring my heart rate back up a bit. I see her in 3 weeks and I'm definitely going to pursue a change.