Avil is the brand name of a medicine called pheniramine maleate. Its maximum permitted dose is 150mg for a person weighing 50 kg or so. Beyond this, it can cause toxicity.
Long time use (abuse) of Avil can lead to agitational behaviour, convulsions (fits), very high fever, blurred vision, disorientation, incoordination of movements, anaemia, urinary retention, development of glaucoma and life threatening agranulocytosis (total absence of white blood cells) when the body cannot fight infection.
Such side effects can come at any time.