affective / effective
Having to do with feeling versus producing a result.
Affective means relating to emotion or mood: affective disorder, affective response. Effective means producing the intended result, or in force: an effective remedy, effective Tuesday. Do not use affective when you mean that something works.
Examples
UseThe treatment was effective within a week.
UseThe film’s affective power comes from the last scene.
AvoidThe treatment was affective within a week.