Pffe... Just make a salad and make sure you put only domesticated white buttons in yours and Amanita bisporigera in theirs. Use different colored bowls if it will help you keep track of whose is whose. Or give them pokeweed in their salad, but make sure yours is boiled properly and theirs isn't.

Or you could do the neat trick of feeding the victim doses of powdered arsenic over a period of time so that it builds up in their tissues and then *stop* feeding them the arsenic. Their body would dump the build-up and poison them that way. You could do it by giving them 'powdered' donuts.

Or inject thallium into their diet sodas. Those things taste funny anyway, and since you only drink regular soda that explains why you didn't get sick AND why you tossed the diet soda after they died.

Thanks to Sisters in Crime, I could probably come up with a good dozen ways to poison someone.