Is it right for a doctor to prescribe a drug even if that drug has nothing to do with the illness ie cymbalta (duloxetine) for premature ejaculation?

Cymbalta (duloxetine): It would be best if you asked the doctor who prescribed this medication to you for an answer to your question. The doctor should go over your concerns with you, and explain the rationale for the medication used to treat the disorder you're experiencing. Also discuss whether your medication is helping or not with your doctor.

Sometimes: If the doctor understand the physiology of the body and the pharmacology of the drug well, off label use of medications is not uncommon. With cymbalta (duloxetine) as the drug it ups serotonin which can slow down ejaculations, but you may notice your libido diminish too.

Medication uses: Some physicians use medication off label meaning for a problem for which they are not approved . They usually have had some experience with the medication and the particular problem With Cymbalta (duloxetine) it may cause ejaculatory disfunction which may help premature ejaculation

Here is one of the: studies your doctor may have reviewed before choosing Cymbalta as your medication: http://www.ncbi.nlm.nih.gov/m/pubmed/17268894/. It concluded that duloxetine, branded Cymbalta, " appears to be superior to placebo in the pharmacological treatment of PE when administered on a chronic basis." I bet if you ask, your doctor will provide more evidence of its efficacy for PE.