For one month the staff room became a lab: seven bestselling whitening products, a box of extracted teeth, and a spectrophotometer that does not do diplomacy.

Two products produced a measurable, stable shade change. One produced exactly nothing, expensively. And one etched enamel badly enough that we filed a report.

Names, numbers and the full shade charts are inside. Bring this article to any pharmacy aisle and it pays for itself.