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.
Modern rat poison is not an element. It is one of several possible organic compounds with anticoagulantproperties.
arsenic
bromide
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
It was chlorine. That's the only poisonous gas that is a pure element that was used in WWI.
Magnesium is used for fireworks and medicines
The common poison gas of WWI was Mustard Gas, a combination of Sulfur and Chlorine (C4H8Cl2S).
The seeds of the apple are poisonus. The poison is cyanide. This is the same poison Hitler used to go suicide.
Barium is used for rat poison.
Mercury is an element and is very poisonous. It is not used as a drug.
Chlorine, which was used as a poisonous gas in World War I.
Lithium