Nail biting has been shown to respond well to certain types of medication. The medications used to treat the problem include the newest, most potent anti-depressants. These medications are also used to treat Trichotillomania and OCD and include clomipramine, fluoxetine (prozac), sertraline, paroxetine, fluvoxamine, citalopram, escitalopram, nefazodone and venlafaxine.
These new generation antidepressants - the selective serotonin reuptake inhibitors - are really safe. I'm on them myself.
You can also try some behaviour therapy like habit reversal training and with Exposure and Response Prevention which some nail biters find helpful.
Hope that helps!