I'm looking for a well-known phrase/idiom/quote/expression to express the meaning that something is good for you but only when you can handle it.
For example, some medicine can cure a disease but only when the person being treated is otherwise healthy enough. If the patient is weak, the medicine may do more harm than good.
I'm struggling to find a well-known and concise way of expressing this. Any suggestions are welcome.
UPDATE: Can I offer another example?
Imagine there is a competition and you can decide either you participate in it alone or together with a friend as a team. Now, having a team member can increase your winning probability only when yourself is strong. Otherwise, your friend will drag you down.