Was just prescribed ketoprofenfor back pain will it interfear with venlafaxune 150 mg?

No. It should be fine. A pain med called tramadol or ultram will raise flags because it has a little bit of serotonin property like venlafaxine, but overall it's fine as well in small amounts.
Yes. Bad Combo. The prior answer is totally wrong! The combination is dangerous because the anti platelet effect of Ketoprofen is exacerbated by the serotonin reuptake blockage caused by venlafaxine.
Bleeding. The combo may increase the risk of bleeding. The NSAID antiplatelet effects are augmented by inhibition of platelet serotonin uptake. Caution advised.