Is it permissible to give the merit of a mitzvah to the benefit of a sick parent so that they should have a refuah shleimah?
I know it mentions this in the gemara about one giving tzedaka so that someone should live, but what about other mitzvos?
Is there a particular mitzvah which is more potent for healing than others?
The reward for mitzvos, generally cannot be given over to other people, because the reward for the mitzva is the purity that the mitzva gives to the person’s neshoma. This purity cannot be given to others. However, if you do extra mitzvos, in order that it should be a zechus for your parent, since it is because of your parent that you are doing this mitzva, they would be considered like assisting you to do the mitzva, and that sechar would benefit them. This idea applies not only to tzedakah but to other mitzvos as well. For example, by doing extra chesed, being nice to other people, learning extra torah, so that it should be a zechus for a refuah shleima, will also help.
The mitzva with the greatest reward and merit is torah learning, as it is equal to all other mitzvos combined. If you will learn some extra, for their recovery, it is a great merit. Saying tehillim is also a great zechus, as it is davening and considered learning.
Hashem should send your parent a refuah shleima, b’karov!