Complex sentence: Tramadol is well-established as a serotonin reuptake inhibitor, and several studies have found that it also acts as a serotonin releasing agent (1--10 μM), similar in effect to fenfluramine.
Split into simpler sentences: Tramadol is well-established as a serotonin reuptake inhibitor. In addition, a few studies have found that it also acts as a serotonin releasing agent (1--10 μM), similar in effect to fenfluramine.