My murderer is a medical intern with access to pharmaceuticals. She tampers with her victim's prescribed medication (probably in capsule form, though I'd take liquid if it would work; definitely should be a form that's possible to ingest). She adds another drug that shouldn't be fatal in and of itself (and thus is unlikely to be looked for in blood work/in a tox screen), but which in combination with the prescribed medication causes a massive stroke.
The victim is a 72-year-old fairly healthy male.
I've Googled "drug interactions," "drug interaction stroke," and various combinations of similar words. I'm not coming up with much except various forms of adrenaline/epinephrine, etc.
Time is present day; location is the United States (Las Vegas, to be precise).