Poisons don't have to take many weeks or months. There are plenty which can kill easily and swiftly without visiting a pharmacist. As explained in my post to Caroline.
Poison virtually always says foul play especially if you poison more than 1 person. The only hope to get away with poisoning is to hope it is mistaken for a natural ailment and that goes out the window if you kill 5 people.
No one would believe June decided to commit murder suicide and intentionally poisoned everyone.
When police do find out poison was used they often track down where it came from which leads to the culpret particularly fast acting poisons. Something that will kill very quickly before any medical treatment can be used to try to save them is not that easy to come by anonymously.