The cumulative effect is on the teeth and gum. Although the effect may not be visible instantly, it would eventually come to light. The effect of brushing too hard is relative from user to user. While some have really tough teeth and gum, others have very fragile and sensitive ones that react quickly. The bristle of your brush also determines how hard it will brush the teeth when pressure is applied. Even when brushed hard, a soft bristle will do little damage to the teeth or gum, while a typically harder bristle will wreak more havoc.