Can common side-effects develop at any time?

Factual Questions
1

I have terrible allergies, so I end up taking a lot of Benadryl. Benadryl doesn’t make me drowsy, which seems to be an unusual reaction. In fact, I will occasionally have trouble sleeping if I take it too late at night.*

But I’m wary about taking it during the day or before driving just in case it does start making me drowsy. Do things like this happen? Can a drug’s common side-effect pop up even if you have never had a problem before? Or can I count on Benadryl not making me drowsy?

*I have had this reaction to all medications I’ve ever had with sedating properties. I don’t know if it’s a coincidence or I’m just weird. Anti-depressants, on the other hand, send me to sleep like crazy.

2

IANAD, but yes. People change and their reactions to meds change over time as well. No sites, just personal experience. I used to be able to use Advil-PM to put me soundly to sleep. Then I quite using it for several months. Now if I use it, it is as likely to give me RLS as it is to make me sleep.

Allergies suck. Luckily I have few.