diff --git a/src/lib/medications/side-effects/__tests__/taxonomy.test.ts b/src/lib/medications/side-effects/__tests__/taxonomy.test.ts
new file mode 100644
index 000000000..3defcfdcf
--- /dev/null
+++ b/src/lib/medications/side-effects/__tests__/taxonomy.test.ts
@@ -0,0 +1,185 @@
+/**
+ * v1.4.25 W19d — taxonomy pure-module tests.
+ *
+ * The taxonomy is a contract the API validator, the picker UI, and
+ * the Coach snapshot all depend on. These tests pin the shape:
+ *   - every entry maps to exactly one category
+ *   - exactly 21 entries across 5 categories
+ *   - every category has at least one entry
+ *   - severity labels are deterministic and ordered
+ */
+
+import { describe, expect, it } from "vitest";
+
+import {
+  MedicationSideEffectCategory,
+  MedicationSideEffectEntry,
+} from "@/generated/prisma/client";
+import {
+  SIDE_EFFECT_CATEGORIES,
+  SIDE_EFFECT_CATEGORY_ORDER,
+  SIDE_EFFECT_ENTRIES_BY_CATEGORY,
+  SIDE_EFFECT_ENTRY_COUNT,
+  SIDE_EFFECT_SEVERITY_LADDER,
+  categoryForEntry,
+  entriesByCategory,
+  isSideEffectSeverity,
+  severityLikertLabel,
+} from "../taxonomy";
+
+const ALL_ENTRIES: MedicationSideEffectEntry[] = [
+  "NAUSEA",
+  "VOMITING",
+  "DIARRHEA",
+  "CONSTIPATION",
+  "ABDOMINAL_PAIN",
+  "HYPOGLYCEMIA_SYMPTOMS",
+  "DEHYDRATION",
+  "ANOREXIA",
+  "ELECTROLYTE_FATIGUE",
+  "INJECTION_REDNESS",
+  "INJECTION_SWELLING",
+  "INJECTION_BRUISING",
+  "INJECTION_INDURATION",
+  "BRAIN_FOG",
+  "DIZZINESS",
+  "LOW_MOOD",
+  "LOW_ENERGY",
+  "EARLY_SATIETY",
+  "GASTROPARESIS_LIKE",
+  "DYSGEUSIA",
+  "GALLBLADDER_DISCOMFORT",
+];
+
+const ALL_CATEGORIES: MedicationSideEffectCategory[] = [
+  "GI",
+  "METABOLIC",
+  "INJECTION_SITE",
+  "COGNITIVE",
+  "GLP1_SPECIFIC",
+];
+
+describe("SIDE_EFFECT_CATEGORIES — entry → category mapping", () => {
+  it("declares exactly 21 entries", () => {
+    expect(Object.keys(SIDE_EFFECT_CATEGORIES)).toHaveLength(21);
+    expect(SIDE_EFFECT_ENTRY_COUNT).toBe(21);
+  });
+
+  it("maps every entry to a known category", () => {
+    for (const entry of ALL_ENTRIES) {
+      const cat = SIDE_EFFECT_CATEGORIES[entry];
+      expect(ALL_CATEGORIES).toContain(cat);
+    }
+  });
+
+  it("categoryForEntry returns the same mapping as the table", () => {
+    for (const entry of ALL_ENTRIES) {
+      expect(categoryForEntry(entry)).toBe(SIDE_EFFECT_CATEGORIES[entry]);
+    }
+  });
+});
+
+describe("SIDE_EFFECT_ENTRIES_BY_CATEGORY — category → entries reverse index", () => {
+  it("covers all five categories", () => {
+    expect(Object.keys(SIDE_EFFECT_ENTRIES_BY_CATEGORY).sort()).toEqual(
+      [...ALL_CATEGORIES].sort(),
+    );
+  });
+
+  it("every category has at least one entry", () => {
+    for (const category of ALL_CATEGORIES) {
+      expect(SIDE_EFFECT_ENTRIES_BY_CATEGORY[category].length).toBeGreaterThan(
+        0,
+      );
+    }
+  });
+
+  it("matches the EMA-derived counts (GI 5, METABOLIC 4, INJECTION_SITE 4, COGNITIVE 4, GLP1_SPECIFIC 4)", () => {
+    expect(SIDE_EFFECT_ENTRIES_BY_CATEGORY.GI).toHaveLength(5);
+    expect(SIDE_EFFECT_ENTRIES_BY_CATEGORY.METABOLIC).toHaveLength(4);
+    expect(SIDE_EFFECT_ENTRIES_BY_CATEGORY.INJECTION_SITE).toHaveLength(4);
+    expect(SIDE_EFFECT_ENTRIES_BY_CATEGORY.COGNITIVE).toHaveLength(4);
+    expect(SIDE_EFFECT_ENTRIES_BY_CATEGORY.GLP1_SPECIFIC).toHaveLength(4);
+  });
+
+  it("reverse index is consistent with the forward mapping", () => {
+    for (const category of ALL_CATEGORIES) {
+      for (const entry of SIDE_EFFECT_ENTRIES_BY_CATEGORY[category]) {
+        expect(SIDE_EFFECT_CATEGORIES[entry]).toBe(category);
+      }
+    }
+  });
+
+  it("every entry appears in exactly one category bucket", () => {
+    const seen = new Set<MedicationSideEffectEntry>();
+    for (const category of ALL_CATEGORIES) {
+      for (const entry of SIDE_EFFECT_ENTRIES_BY_CATEGORY[category]) {
+        expect(seen.has(entry)).toBe(false);
+        seen.add(entry);
+      }
+    }
+    expect(seen.size).toBe(21);
+  });
+
+  it("entriesByCategory helper is a thin wrapper", () => {
+    for (const category of ALL_CATEGORIES) {
+      expect(entriesByCategory(category)).toEqual(
+        SIDE_EFFECT_ENTRIES_BY_CATEGORY[category],
+      );
+    }
+  });
+});
+
+describe("SIDE_EFFECT_CATEGORY_ORDER — UI category sequence", () => {
+  it("contains every category exactly once", () => {
+    expect([...SIDE_EFFECT_CATEGORY_ORDER].sort()).toEqual(
+      [...ALL_CATEGORIES].sort(),
+    );
+  });
+
+  it("leads with GI — the EMA most-common cluster during titration", () => {
+    expect(SIDE_EFFECT_CATEGORY_ORDER[0]).toBe("GI");
+  });
+});
+
+describe("severityLikertLabel — 1-5 Likert → semantic label", () => {
+  it("returns mild / moderate / significant / severe / verySevere", () => {
+    expect(severityLikertLabel(1)).toBe("mild");
+    expect(severityLikertLabel(2)).toBe("moderate");
+    expect(severityLikertLabel(3)).toBe("significant");
+    expect(severityLikertLabel(4)).toBe("severe");
+    expect(severityLikertLabel(5)).toBe("verySevere");
+  });
+
+  it("is deterministic — same input twice yields the same output", () => {
+    expect(severityLikertLabel(3)).toBe(severityLikertLabel(3));
+  });
+
+  it("ladder is monotonic — index matches severity - 1", () => {
+    for (let s = 1; s <= 5; s++) {
+      expect(SIDE_EFFECT_SEVERITY_LADDER[s - 1]).toBe(
+        severityLikertLabel(s as 1 | 2 | 3 | 4 | 5),
+      );
+    }
+  });
+
+  it("ladder length is 5", () => {
+    expect(SIDE_EFFECT_SEVERITY_LADDER).toHaveLength(5);
+  });
+});
+
+describe("isSideEffectSeverity — type guard", () => {
+  it("accepts 1 through 5", () => {
+    expect(isSideEffectSeverity(1)).toBe(true);
+    expect(isSideEffectSeverity(5)).toBe(true);
+  });
+
+  it("rejects 0 and 6", () => {
+    expect(isSideEffectSeverity(0)).toBe(false);
+    expect(isSideEffectSeverity(6)).toBe(false);
+  });
+
+  it("rejects non-integer", () => {
+    expect(isSideEffectSeverity(2.5)).toBe(false);
+  });
+});
diff --git a/src/lib/medications/side-effects/taxonomy.ts b/src/lib/medications/side-effects/taxonomy.ts
new file mode 100644
index 000000000..7948cd6db
--- /dev/null
+++ b/src/lib/medications/side-effects/taxonomy.ts
@@ -0,0 +1,196 @@
+/**
+ * v1.4.25 W19d — GLP-1 side-effect taxonomy (pure module).
+ *
+ * Single source of truth for the entry → category mapping the picker
+ * UI, the API validator, and the Coach snapshot all read. Derived
+ * from EMA EPAR §4.8 ("Undesirable effects") for tirzepatide /
+ * semaglutide / liraglutide / dulaglutide, clustered into HealthLog's
+ * five surface categories:
+ *
+ *   - GI                — gastrointestinal (very common during titration)
+ *   - METABOLIC         — hypo / dehydration / appetite-suppression edges
+ *   - INJECTION_SITE    — local-site reactions (EMA "common")
+ *   - COGNITIVE         — brain-fog / dizziness / mood / energy (HealthLog
+ *                         keeps these on the medication record because
+ *                         the user wants the drug-correlation surface;
+ *                         the existing MoodEntry stream is independent).
+ *   - GLP1_SPECIFIC     — the four drug-mechanism signatures (early
+ *                         satiety, gastroparesis-like fullness, taste
+ *                         change, gallbladder discomfort).
+ *
+ * The DB stores `category` denormalised so range / category-filter
+ * queries don't need a JOIN, but this module is the authority — the
+ * API write path derives the category from the entry instead of
+ * trusting the client.
+ *
+ * Severity is 1-5 Likert. The semantic-label helper returns a
+ * locale-agnostic ladder key (`mild` → `verySevere`); the UI maps each
+ * key to a translated label.
+ */
+
+import {
+  MedicationSideEffectCategory,
+  MedicationSideEffectEntry,
+} from "@/generated/prisma/client";
+
+export const SIDE_EFFECT_CATEGORIES: Record<
+  MedicationSideEffectEntry,
+  MedicationSideEffectCategory
+> = {
+  // GI (5)
+  NAUSEA: "GI",
+  VOMITING: "GI",
+  DIARRHEA: "GI",
+  CONSTIPATION: "GI",
+  ABDOMINAL_PAIN: "GI",
+  // Metabolic (4)
+  HYPOGLYCEMIA_SYMPTOMS: "METABOLIC",
+  DEHYDRATION: "METABOLIC",
+  ANOREXIA: "METABOLIC",
+  ELECTROLYTE_FATIGUE: "METABOLIC",
+  // Injection-site (4)
+  INJECTION_REDNESS: "INJECTION_SITE",
+  INJECTION_SWELLING: "INJECTION_SITE",
+  INJECTION_BRUISING: "INJECTION_SITE",
+  INJECTION_INDURATION: "INJECTION_SITE",
+  // Cognitive (4)
+  BRAIN_FOG: "COGNITIVE",
+  DIZZINESS: "COGNITIVE",
+  LOW_MOOD: "COGNITIVE",
+  LOW_ENERGY: "COGNITIVE",
+  // GLP-1 specific (4)
+  EARLY_SATIETY: "GLP1_SPECIFIC",
+  GASTROPARESIS_LIKE: "GLP1_SPECIFIC",
+  DYSGEUSIA: "GLP1_SPECIFIC",
+  GALLBLADDER_DISCOMFORT: "GLP1_SPECIFIC",
+};
+
+/**
+ * Total entry count exposed as a const so callers (validators,
+ * snapshot aggregators) can assert against drift without re-counting.
+ */
+export const SIDE_EFFECT_ENTRY_COUNT = 21;
+
+/**
+ * Ordered category list — defines the picker UI's category-tab
+ * sequence. Stable across versions; new categories append.
+ */
+export const SIDE_EFFECT_CATEGORY_ORDER: readonly MedicationSideEffectCategory[] =
+  [
+    "GI",
+    "METABOLIC",
+    "INJECTION_SITE",
+    "COGNITIVE",
+    "GLP1_SPECIFIC",
+  ] as const;
+
+/**
+ * Entries grouped under each category in picker-UI order. The order
+ * within a category mirrors the EMA EPAR §4.8 listing where one
+ * exists, then EMA-derived HealthLog clustering for the rest.
+ */
+export const SIDE_EFFECT_ENTRIES_BY_CATEGORY: Record<
+  MedicationSideEffectCategory,
+  readonly MedicationSideEffectEntry[]
+> = {
+  GI: ["NAUSEA", "VOMITING", "DIARRHEA", "CONSTIPATION", "ABDOMINAL_PAIN"],
+  METABOLIC: [
+    "HYPOGLYCEMIA_SYMPTOMS",
+    "DEHYDRATION",
+    "ANOREXIA",
+    "ELECTROLYTE_FATIGUE",
+  ],
+  INJECTION_SITE: [
+    "INJECTION_REDNESS",
+    "INJECTION_SWELLING",
+    "INJECTION_BRUISING",
+    "INJECTION_INDURATION",
+  ],
+  COGNITIVE: ["BRAIN_FOG", "DIZZINESS", "LOW_MOOD", "LOW_ENERGY"],
+  GLP1_SPECIFIC: [
+    "EARLY_SATIETY",
+    "GASTROPARESIS_LIKE",
+    "DYSGEUSIA",
+    "GALLBLADDER_DISCOMFORT",
+  ],
+} as const;
+
+/**
+ * Lookup helper — returns the entries for a category. Used by the
+ * picker UI to filter the chip list when the user changes category.
+ */
+export function entriesByCategory(
+  category: MedicationSideEffectCategory,
+): readonly MedicationSideEffectEntry[] {
+  return SIDE_EFFECT_ENTRIES_BY_CATEGORY[category];
+}
+
+/**
+ * Lookup helper — returns the category for an entry. The API write
+ * path uses this instead of trusting a client-supplied category, so a
+ * mismatched payload never lands in the DB.
+ */
+export function categoryForEntry(
+  entry: MedicationSideEffectEntry,
+): MedicationSideEffectCategory {
+  return SIDE_EFFECT_CATEGORIES[entry];
+}
+
+/** 1-5 Likert severity scale. */
+export type SideEffectSeverity = 1 | 2 | 3 | 4 | 5;
+
+/** Locale-agnostic semantic labels for the severity ladder. */
+export type SideEffectSeverityLabel =
+  | "mild"
+  | "moderate"
+  | "significant"
+  | "severe"
+  | "verySevere";
+
+/**
+ * Maps a 1-5 severity integer to its locale-agnostic semantic label.
+ * The UI looks up the translated label via
+ * `t("medications.sideEffects.severity.<label>")`.
+ *
+ * Ordering is fixed: mild < moderate < significant < severe <
+ * verySevere. Tests assert determinism + ordering.
+ */
+export function severityLikertLabel(
+  severity: SideEffectSeverity,
+): SideEffectSeverityLabel {
+  switch (severity) {
+    case 1:
+      return "mild";
+    case 2:
+      return "moderate";
+    case 3:
+      return "significant";
+    case 4:
+      return "severe";
+    case 5:
+      return "verySevere";
+  }
+}
+
+/**
+ * Ordered severity-label ladder — useful for the picker UI which
+ * renders five Likert buttons in ascending severity order.
+ */
+export const SIDE_EFFECT_SEVERITY_LADDER: readonly SideEffectSeverityLabel[] = [
+  "mild",
+  "moderate",
+  "significant",
+  "severe",
+  "verySevere",
+] as const;
+
+/**
+ * Type guard — narrows a raw number into the SideEffectSeverity union.
+ * The Zod schema enforces this on the wire; the guard is for code
+ * paths that read the persisted integer back.
+ */
+export function isSideEffectSeverity(
+  value: number,
+): value is SideEffectSeverity {
+  return value === 1 || value === 2 || value === 3 || value === 4 || value === 5;
+}