In the field of Mental Health - aside from addicting medicines - most medicines only make some things possible- more like altering the balance of multiple mechanisms.

So it is possible to "fight" the effects of the medicine unless it is very strong. That is one of the reasons that of the old antipscyhotic and antidepressant medicines the ones that worked best were those that had a sleeping side effect. This effectively bypassed the patient fighting the effects of the medicine, as the most effect is in the first few hours.