omo
Well, diphenhydramine has its uses. I wouldn't discount it completely.
For example, if you're having an allergic reaction and risk going into anaphylactic shock, you should be taking an antihistamine like diphenhydramine.
If you're generally high histamine, taking diphenhydramine could provide some relief.
If you're low serotonin, diphenhydramine could make your current serotonin last longer. So it might be useful as a mood enhancer.
You just need to be aware that it may combine dangerously with other things. Diphenhydramine basically makes your serotonin more powerful. You don't want to do that at the same time you increase the amount of serotonin in your system (with, for example, 5-HTP). That would be a bad combination.
Note that St. John's Wort is supposed to have some serotonin reuptake inhibitory effect. Which means you also shouldn't be taking St. John's Wort with 5-HTP.