Can I use Retin-A effectively over facial hair?

Will using Retin-A over facial hair limit its efficacy? I have a modest amount of facial hair, most of which is concentrated on my jaw-line. I use Retin-A mostly on my cheeks for acne, where I have a small amount of facial hair. I prefer to trim my facial hair as I have very sensitive skin, but I was wondering if this will decrease the drug's effectiveness. Also, is there anything I can do (short of shaving) to ensure proper delivery of the drug?