| Metoprolol ER is a salt of succinic acid. Metoprolol Tartrate is a salt of tartaric acid. The Metoprolol part is the same in both cases and the effect on the body is the same.
Manufacturers make salts out of many compounds in order to increase their solubility in the stomach. Metoprolol, itself, has a molecular weight of 267.37. Succinic acid has a molecular weight of 118.09. Tartaric acid has a molecular weight of 150.087. So, you'll get the same amount of Metoprolol in 385 mg of Metoprolol ER or 417 mg of the Metoprolol Tartrate. In other words, it takes 8.3% more of the tartrate to give the same effect on the body. I expect that most physicians will ignore the difference. | 
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