Beta blockers are used to treat a variety of conditions, including high blood pressure and migraines. The exact cause of weight gain associated with some beta blockers isn't clearly understood. But it may be due to a lower metabolic rate and reduced physical activity. In addition, if you switch from a diuretic to a beta blocker, you may gain a few pounds of weight that the diuretic kept off.
It is important to note that weight gain may also be due to fluid retention, which may indicate worsening heart failure. Talk to your doctor if you experience unexplained weight gain soon after starting a beta blocker — especially if you gain more than 3 to 4 pounds (1.3 to 1.8 kilograms). If you have sudden weight gain — a weight gain of 3 to 4 pounds in 24 to 48 hours — consult your doctor promptly.