Poison of the Old Man is a Quick-Play Spell Card.
Spell Cards are usually activated from the hand on your turn unless their type gives another timing window.
On the current TCG and OCG Forbidden & Limited lists tracked here, Poison of the Old Man is Unlimited.
YGOPRODeck lists Poison of the Old Man for these formats: GOAT, OCG GOAT, Duel Links, Common Charity, Edison, TCG, OCG, Master Duel.
A listed print of Poison of the Old Man appears in Dark Revelation Volume 1 (DR1-EN088) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Poison of the Old Man is about $0.14 on Cardmarket (prices change frequently).
