Making a decision requires you to compare tests/treatments that have been contrasted in research studies to see if one over another results in improved chances of good outcomes. In a sense, medical decision making is a competition. To assess the competition, you compare the chances of outcomes, or results from groups of people taking different options. The comparison is a simple subtraction in the amounts of outcomes that occur in each studied group.
Subtracting results in a difference that is either a benefit (if better for you) or a harm (if worse for you). For nearly all decisions, however, the test/treatment that is better for disease outcomes (benefit) is worse for complications (harm). Comparing, then, results in the following possibilities:
The chances of outcomes associated with the condition you have and the tests/treatments available will be the same for all options. In this case, chose the cheapest option.
The chance of outcomes associated with the condition you have will be less with one option. That option provides added benefit
The chance of a complication caused by the test/treatment that adds benefit for the disease outcomes will be greater (harm).
Since the test/treatment that is better for you in terms of the disease you have will be, simultaneously, worse for you in terms of complications caused by that test/treatment, a trade-off of benefit and harm is required.
Hence, the definition of “works” is that:
A test/treatment works when you feel there is more to gain from the greater chance of better disease associated outcomes than there would be to lose from suffering the complications caused by your chosen treatment.
So, medical-decision-making is a competition between options and there is always some good to be balanced against some bad.
The balance of good and bad from your perspective is what makes one treatment work over another.