Complex sentence: Malic acid and sodium bicarbonate are both effective whitening treatments, but should be used sparingly as both methods are not too gentle on the teeth, and could lead to enamel damage if used indiscriminately (i.e. more than a couple of times a week or so).
Split into simpler sentences: Malic acid and sodium bicarbonate are effective whitening treatments but should be used sparingly as both methods are not too gentle on the teeth. They could lead to enamel damage if used indiscriminately (i.e., more than a couple of times a week or so).