Anxiety can be a physiological issue, yes. Many things can cause what seem to be panic attacks, including blood sugar problems, thyroid problems, and other things. But if this was something he went through before and it started when your Dad died, then that's probably the catalyst and what he needs to work on. For that, he'll need to see a psychologist for therapy to learn why he has become so insecure. Losing a parent affects people very differently, but this appears to be how it affected your brother and he needs to learn why. You also say he was using meds and they worked but he stopped them. Did he stop them abruptly, or did he taper off of them as slowly as necessary to prevent withdrawal? Stopping medication too quickly for a particular person can also cause anxiety attacks. It is curious it happens only at a particular time of day. Is that the time of day your Dad died? Why only then? These are things medication can treat symptomatically, but can't fix permanently. But therapy might. In the meantime, if life is not liveable, it is sometimes necessary to take medication to get back to balance, but again, no, there is no medication yet found that permanently fixes emotional problems if that's what this is because science doesn't yet know what or even whether something within the body goes wrong that causes this and if so what it is that's gone wrong. So the best we can do with meds is tamp down the symptoms. For many, that's the best that can be done. But if you know the trigger that started the problem, therapy works better for that than for those who just get these problems out of nowhere, which can be harder to deal with in therapy.