That's a very important point. There are various classes of antihistamines, H1 antagonists and inverse agonists and each can produce profoundly different effects on some people. In addition some, like Benadryl and generic diphenhydramine, are anticholingerics and can produce severe side effects in some people, including sudden depression, disorientation, etc. If at all possible avoid taking any of the
H1 antagonists on this list, which includes most inexpensive over the counter remedies.
Some of the worst are the Dayquil/Nyquil and generic all-purpose cold/flu/allergy meds. Never take those if you can avoid it. Some contain up to three anticholinergics, the combination of which can result in sleep disturbances, nightmares, disorientation, memory loss and depression. These all-in-one meds tend to use the cheapest, nastiest over the counter remedies for allergies, stuffy noses, coughs, etc., all of which have been surpassed by much better OTC choices.
It's almost always better to pinpoint the specific symptoms and take only the remedy for that symptom. No need to take antihistamines if you have a cold or flu with respiratory congestion, runny nose, etc.
I do best with Zyrtec, while Claritin does nothing for me.
If you have a stuffy nose that's severe enough to cause pain, and doesn't respond to mild saline irrigation, steam or good antihistamines, about the only thing that works effectively is real Sudafed or generic pseudoephedrine. Nowadays you have to sign for it in the US (thanks, meth heads).
Forget phenylephrine, the usual generic OTC substitute. It's crap. Studies show it's ineffective as a nasal decongestant. No point taking it and some folks will experience side effects. The one thing phenylephrine is occasionally useful for is hemorrhoid relief. It does provide some relief from swelling of hemorrhoidal tissues, although it isn't effective with sinuses. Not sure why, but research confirms this oddity. If you check the ingredients on some hemorrhoid wipes and ointments it'll contain phenylephrine and witch hazel. You can buy both separately and concoct your own relief ointments.
I could go on and on about this. It's a pet peeve of mine because I've been caregiver for three elderly relatives over the past 20 years and saw each one experience unnecessary and avoidable confusion, disorientation and short term memory loss from taking OTC and prescription meds that were inappropriate for their conditions. Fortunately their doctors were willing to change prescriptions after discussing it, but I'm always disappointed that they don't seem to consider the side effects before prescribing this stuff in the first place.