Efexor XR is (Extended release)
Same as Effexor except it's designed to release gradually over a 6 to 8 hour period.
It was developed a while after Effexor because some patients complained that the release was too rapid. The dosages are generally the same. The active ingredient "Venlafaxine" is identical in every way in both Effexor and Effexor XR.
Only the actual delivery method is different.
I have taken both and I prefer the XR because with the XR it really doesn't matter what time of day or night you take it. Some people also report less side effects with the XR variant.
I was on the regular effexor, and I would not reccommend it to anyone else. Severe side effects and disturbing withdrawal effects if you don't take it at the same time each day, or worse, miss a day. The withdrawal effects render you almost non-functional. Effexor ER would be much more manageable and less likely to interfere in your life. Better yet, ask the dr. for a different anti-depressant. There are many more that are just as effective, or more effective.