A quantity can be increased by any percentage but should not be decreased by more than 100 percent.

Once sales have been reduced by 100 percent, for example, it ceases to exist. In defiance of this logic, however, advertisers sometimes refer to a 150 percent decrease in new luggage prices or a new medication that increases energy by over 300 percent.

It is assumed that what is implied by the last example is that the new medicine is three times as effective as some other medicine, but such constructions are still considered to be etymologically incorrect because 100 percent is the
total amount of anything.