I seem to have my sense of smell being blocked, it ocasionally comes and then i can't smell anything. its been like that for 5 years . is it allergie?

Frequently: The smell receptors are located in the very top of the nasal cavity. If there is any swelling in the nose, or obstruction of the nose due to polyps, air may not be able to reach the sensors, and you won't be able to smell. If you have an intermittent loss of smell, that is more likely to be allergy, rather than an obstruction that never changes, like a septal deviation.

Possibly: Diminished sense of smell that is intermittent is likely due to poor airflow to the roof of the nose where the nerve endings for smell are located. A thorough nasal endoscopy will check for nasal polyps ( which are usually due to allergies). If the endoscopy is normal, then a ct scan may be helpful in ensuring no other abnormalities in the skull base that can decrease sense of smell.