Pharmacodynamics is merely the study of a drug's effects on a living organism. With many drugs, the system develops a tolerance with repeated use, meaning the dosage has to be increased in order to achieve the same pharmacological result, or a different type of drug needs to be introduced in order to produce a similar result. For example, heroin users often report that the "high" they get from the drug is never as good as the very first time they took it. They spend their addiction trying to "chase" the original effect of the drug as their system becomes increasingly tolerant of it.
When deciding which drugs to prescribe, or determining the dosage to prescribe, tolerance has to be taken into account. Someone who takes Vicodin (an Opiate) for a long period of time for pain relief may need a different type of drug altogether to continue relieving pain so that addiction does not occur, and so that the relief continues ot be effective. If side effects are not a concern, then dosage needs to be continually adjusted so that, once again, the medical objective can be received.