Metoprolol comes in two formulations,
Metoprolol Tartrate and Metoprolol Succinate. If you were prescribed the Tartrate variety, 50mg, once a day as you stated, then you were on a beta blocker roller coaster with a steep peak followed by a deep valley. Metoprolol has a rather short half life of approximately 6 hours, so in 24 hours, 50mg is nearly completely metabolized. This type should be taken twice a day.