> Valid ad hominem arguments occur in informal logic, where the person making the argument relies on arguments from authority such as testimony, expertise, or on a selective presentation of information supporting the position they are advocating. In this case, counter-arguments may be made that the target is dishonest, lacks the claimed expertise, or has a conflict of interest.
For example, if someone tells you that hydroxychloroquinea will cure COVID and cites a doctor, it is an ad hominem not but not a fallacy to counter that the same doctor also says that infertility, impotence, cysts, and various other reproductive medical problems are caused by witches and demons that have sex with people in the dreamworld, where they also gather sperm from people and use it on other people to produce more demons. (And yes, there really is a doctor who says all that).
It's not a fallacy because it is not offered to refute the claim that hydroxychloroquine cures COVID--it is offered to show that the person making the claim is not competent to make the claim.