rachel,
All beta-blockers have this very undesirable side effect...I gained nearly 10 pounds in 14 days on atenolol

and needed to squeeze EXTREMELY hard on the calories to reverse the trend!
After 30 days I jettisoned the drug.
The cause and effect is very easy to see since beta-blockers work by slowing the heart rate and force of each pulse (along with OTHER muscles unfortunately)...but just the cardiac effect is enough to cause the most energy intensive organ of the body to drastically lower it's caloric needs. AND WORSE the favored fuel of cardiac muscle is teeny bits of fat molecules (acyl-CoA.)
This class of drugs is probably the dieter's worst enemy (well ALMOST ...nothing beats PREDNISONE for porking-up.)