Metoprolol is available as two different salts: metoprolol tartrate and metoprolol succinate.
The main difference between metoprolol tartrate and metoprolol succinate is that metoprolol tartrate is only available as an immediate-release tablet which means it must be taken several times per day, whereas metoprolol succinate is an extended-release tablet that can be taken once a day. As a result, there are differences in the dosages and indications for both metoprolol tartrate and metoprolol succinate and they are not considered interchangeable.