There are many reasons why your teeth darken or stain they fall into 3 categories,

Extrinsic - staining on the outside enamel surface, builds up in biofilm that covers the tooth.

Intrinsic - staining caused by something inside the tooth, or body, often occurs during the formation of enamel or as result of damage/trauma to the tooth.

Age related changes – these happen later in life as the transparent enamel thins and the darker dentine shines through, dentine also darkens as it matures.

Intrinsic and age-related discolouration requires professional dental bleaching to improve appearance safely and effectively.  Extrinsic is removed by the Ohh! Flow.