So I'm running a 5e game in a homebrew world. This world is very tied into different Gods and Clerics are readily available. Arcane magic is also a day-to-day occurrence for most of the citizens, even if it's something relatively minor. There was recently a plot point that was pursued that brought the player characters to the home of their Fighter, who has an ailing father, a nobleman of advanced years. 

This led me to a thought: How does someone in a world where magic is commonplace poison a man who has both the means for treatment, and would have access to it? Any suggestions would be appreciated.

Arsenic.  Small doses on a regular basis.  Gradually increase the size of the dosage.  Once it's reached a certain point, stop. 

I don't remember the rest of the details off the top of my head. 

Okay, I remembered a little more.  You start with a very small dose and work your way up to what would be a lethal dose, but by that time the victim has built up a tolerance to it.  Then you stop and it all hits them at once, if I remember correctly.