The Celexa molecule (chemical name citralopram) comes in a right-handed and left-handed version just as gloves are handed. They figured out that only the left-handed version was useful for depression and separated it from the right-handed version (which only causes side effects). The left-handed version by itself is called Lexapro (escitralapram - es standing for left) and is exactly half the dose of Celexa because Celexa is 50% left and right-handed. Leaving out the right-handed molecules is thought to eliminate some side effects. I don't know if this means less sexual side effects with Lexapro though, the sexual side effects are related to serotonin. There is no long-term evidence of weight gain with either, as neither stimulate the H1 histamine receptors in the brain responsible for increased appetite.