diff --git a/src/combat.test.js b/src/combat.test.js
index c887572..996c755 100644
--- a/src/combat.test.js
+++ b/src/combat.test.js
@@ -1,6 +1,7 @@
 import { beforeAll, describe, expect, test } from "bun:test";
 import { initCards } from "./cards.js";
 import { checkCombatEnd, resolveEnemyTurn, startTurn } from "./combat.js";
+import { resolveEffects } from "./effects.js";
 import { initEnemies } from "./enemies.js";
 import { createCombatState } from "./state.js";
 
@@ -144,3 +145,80 @@ describe("checkCombatEnd - multiplayer", () => {
     expect(checkCombatEnd(state)).toBe("defeat");
   });
 });
+
+describe("poison effect", () => {
+  test("applying poison adds poison to target enemy", () => {
+    let state = createCombatState("ironclad", "jaw_worm");
+    state = {
+      ...state,
+      enemies: [{ ...state.enemies[0], poison: 0 }],
+      enemy: { ...state.enemy, poison: 0 },
+    };
+    const next = resolveEffects(
+      state,
+      [{ type: "poison", value: 3 }],
+      { type: "player", index: 0 },
+      { type: "enemy", index: 0 },
+    );
+    expect(next.enemies[0].poison).toBe(3);
+  });
+
+  test("poison stacks additively up to 30", () => {
+    let state = createCombatState("ironclad", "jaw_worm");
+    state = {
+      ...state,
+      enemies: [{ ...state.enemies[0], poison: 0 }],
+      enemy: { ...state.enemy, poison: 0 },
+    };
+    let next = resolveEffects(
+      state,
+      [{ type: "poison", value: 20 }],
+      { type: "player", index: 0 },
+      { type: "enemy", index: 0 },
+    );
+    next = resolveEffects(
+      next,
+      [{ type: "poison", value: 15 }],
+      { type: "player", index: 0 },
+      { type: "enemy", index: 0 },
+    );
+    expect(next.enemies[0].poison).toBe(30);
+  });
+});
+
+describe("poison tick", () => {
+  test("poison damages enemy and decrements at start of enemy turn", () => {
+    let state = createCombatState("ironclad", "jaw_worm");
+    const poisonedEnemy = { ...state.enemies[0], poison: 5, hp: 10 };
+    state = {
+      ...state,
+      enemies: [poisonedEnemy],
+      enemy: poisonedEnemy,
+      combat: { ...state.combat, dieResult: 1 },
+    };
+    const next = resolveEnemyTurn(state);
+    expect(next.enemies[0].hp).toBe(5);
+    expect(next.enemies[0].poison).toBe(4);
+  });
+
+  test("enemy dies from poison", () => {
+    let state = createCombatState("ironclad", "jaw_worm");
+    const poisonedEnemy = { ...state.enemies[0], poison: 10, hp: 3 };
+    state = {
+      ...state,
+      enemies: [poisonedEnemy],
+      enemy: poisonedEnemy,
+      combat: { ...state.combat, dieResult: 1 },
+    };
+    const next = resolveEnemyTurn(state);
+    expect(next.enemies[0].hp).toBe(0);
+    expect(next.enemies[0].poison).toBe(9);
+  });
+
+  test("zero poison does not tick", () => {
+    let state = createCombatState("ironclad", "jaw_worm");
+    state = { ...state, combat: { ...state.combat, dieResult: 1 } };
+    const next = resolveEnemyTurn(state);
+    expect(next.enemies[0].poison).toBe(0);
+  });
+});
diff --git a/src/effects.js b/src/effects.js
index be32590..bb6520a 100644
--- a/src/effects.js
+++ b/src/effects.js
@@ -71,6 +71,8 @@ function resolveSingleEffect(state, effect, source, target) {
       return applyStatus(state, target, "vulnerable", effect.value, 3);
     case "weak":
       return applyStatus(state, target, "weak", effect.value, 3);
+    case "poison":
+      return applyStatus(state, target, "poison", effect.value, 30);
     case "strength":
       return applyStatus(state, source, "strength", effect.value, 8);
     case "draw": {
diff --git a/src/state.js b/src/state.js
index 7b87730..2a42331 100644
--- a/src/state.js
+++ b/src/state.js
@@ -14,6 +14,7 @@ function makePlayer(character, index) {
     strength: 0,
     vulnerable: 0,
     weak: 0,
+    poison: 0,
     drawPile: shuffle([...getStarterDeck(character)]),
     hand: [],
     discardPile: [],
@@ -34,6 +35,7 @@ function makeEnemy(enemyId, index) {
     strength: 0,
     vulnerable: 0,
     weak: 0,
+    poison: 0,
     actionType: enemy.actionType,
     actions: enemy.actions,
     actionTrack: enemy.actionTrack || null,
@@ -88,6 +90,7 @@ export function createCombatFromRun(run, enemyIdOrIds) {
     strength: 0,
     vulnerable: 0,
     weak: 0,
+    poison: 0,
     drawPile: shuffle([...run.deck]),
     hand: [],
     discardPile: [],