It's a fantasy setting, so I have some leeway, but I at least want to know the basics before I use this as a plot device.
Search terms use: "lead," "lead poisoning symptoms," "chronic lead poisoning symptoms," "lead poisoning symptoms onset," "lead poisoning symptoms delay"
----->EDIT: I like the idea of using lead poisoning because its symptoms (weakness and tingling in the extremities, headaches, loss of coordination, and a leaden pallor) all have an ominous feel without seeming immediately life-threatening.
The poisoner's goal is to remove the character as a romantic rival. If the character thinks he is cursed and must leave to have the curse lifted, then all is well. If his symptoms suddenly disappear, he would come back home as fast as he could. The last thing the poisoner would do is follow his victim to continue administering poison.
I do like the idea of tricking the character into poisoning himself, but I want to make sure the method is gradual and would not have him suddenly running out of doses of poison halfway through his journey. I'll give it some more thought.
Thanks for the feedback so far! It's proven very informative.