For months now i have had a post nasal drip, that wont go away and causes an annoying cough. i have taken zertec, claritin and benedryl?

Diagnosis: You really need a proper diagnosis of your cough, it may not be nasal allergies, as it can be from sinusitis or even acid reflux. Consult your doctor or see an allergist, they can test you and help you identify the source of your symptoms. You may need a nasal spray to control the drip if the oral antihistamines don't help and it is indeed allergies you are suffering from.