Several.
Arsenic was once used as a rat poison and also has use in medicine (bone marrow disorders as an example.
Thallium was once used as a rat poison and was once used to treat Syphillis
Barium used to be used in rat poison and is used in medicine (barium meal) for some types of scan.
Mostly these days however it is a compound that applies to both rather than an element - that being Warfarin.
bromide
Arsenic is the poisonous element used in both medicines and rat poison. In controlled doses, arsenic is used in certain medications to treat certain medical conditions, while in larger quantities it is used as a rodenticide to kill rats and other pests.
A medicine that is poisonous, but was once used for medical treatment is Warfarin or Coumadin. These are both medicines used to thin the blood, but are also used in rat poison. Ether is also poisonous and was used in surgery to put a patient to sleep.
an element that i think is a very old poison is mercury Antimony Arsenic Bismuth Mercury Copper Cadmium and Lead are all poisonous and used in old paint
Magnesium is used for fireworks and medicines
The seeds of the apple are poisonus. The poison is cyanide. This is the same poison Hitler used to go suicide.
Mercury is an element and is very poisonous. It is not used as a drug.
Yes, a poison is still poisonous even if it expires. The potency of the poison may decrease over time, but it can still be harmful if ingested or used improperly.
Chlorine, which was used as a poisonous gas in World War I.
Lithium
What poison, specifically? Several elements are poisonous all by themselves ... arsenic, chlorine, and fluorine to name a few. Lead isn't acutely "poisonous", but it (and several other elements) cause heavy metal toxicity. Plutonium will kill you via ionizing radiation. And so on.
iodine