Tweens and teens sometimes feel resistant to taking psychiatric medication even when they feel deeply unhappy or even when they have already experienced a positive result from the medicine. They may see medication as threatening their autonomy (i.e., the medicine is controlling them or their behavior).
Credit your child not the medication: Explain to your child that the medicine is simply giving them the opportunity to do well if they are motivated to do so. Any improvement in the child’s behavior is occurring because of the child’s motivation to do well and not because of the medicine.
Don’t let your moods control you: Turn your child’s fear of being controlled by the medication on its head. Suggest to your child that pre-medicine, they were being controlled by the “tantrums” or “depression” or “bad moods” or “Godzilla” (or whatever word you are using to describe the “problem”). The medicine allowed your child to take back their control freeing themselves to act as they wish.
Medication won’t turn you into a Zombie: Sometimes children fear that medicine will keep them from having a full range of emotion. In that case, you might explain that children (and adults) will sometimes be in a bad mood and taking medication won’t change that. They will still feel their positive and negative feelings and remain perfectly capable of being oppositional, defiant, tantrumy etc. What is different is that if the medicine effective, they will more consistently feel better and when in a bad mood, medication will make it possible for them to choose to act more maturely and thoughtful. Or not.
The goal of medicine is to give the child, rather then the “disorder” control over their behavior. If after taking medication your child succeeds in making positive behavioral changes, lavishly credit your motivated child for the change. The medication did not force them to improve their behavior or focus more effectively. It simply made it possible for them to do what they are motivated to do.