diff --git a/simple-analytics.php b/simple-analytics.php
index 3c253cd..1a12971 100644
--- a/simple-analytics.php
+++ b/simple-analytics.php
@@ -48,6 +48,7 @@
 require __DIR__ . '/src/Actions/AddNoScriptTag.php';
 require __DIR__ . '/src/Actions/AddPluginSettingsLink.php';
 require __DIR__ . '/src/Support/Str.php';
+require __DIR__ . '/src/Support/OnloadCallback.php';
 require __DIR__ . '/src/Settings/Block.php';
 require __DIR__ . '/src/Settings/Blocks/CalloutBlock.php';
 require __DIR__ . '/src/Settings/Concerns/HasDocs.php';
@@ -122,7 +123,7 @@
             ->description('Collect analytics from visitors with disabled or no JavaScript.');
 
         $tab->input(SettingName::ONLOAD_CALLBACK, 'Onload Callback')
-            ->description('JavaScript function to call when the script is loaded.')
+            ->description('JavaScript to run when the script loads. Requires permission to save unfiltered HTML. After upgrading, save an existing callback again to enable it.')
             ->placeholder('Example: sa_event("My event")')
             ->docs('https://docs.simpleanalytics.com/trigger-custom-page-views#use-custom-collection-anyway');
 
diff --git a/src/Scripts/AnalyticsScript.php b/src/Scripts/AnalyticsScript.php
index b769bb9..271235b 100644
--- a/src/Scripts/AnalyticsScript.php
+++ b/src/Scripts/AnalyticsScript.php
@@ -7,6 +7,7 @@
 use SimpleAnalytics\Scripts\Contracts\Script;
 use SimpleAnalytics\Setting;
 use SimpleAnalytics\SettingName;
+use SimpleAnalytics\Support\OnloadCallback;
 
 class AnalyticsScript implements Script, HasAttributes, HideScriptId
 {
@@ -28,7 +29,7 @@ public function attributes(): array
             'data-collect-dnt'  => Setting::boolean(SettingName::COLLECT_DNT) ? 'true' : null,
             'data-ignore-pages' => Setting::get(SettingName::IGNORE_PAGES),
             'data-auto-collect' => Setting::boolean(SettingName::MANUAL_COLLECT) ? 'false' : null,
-            'data-onload'       => Setting::get(SettingName::ONLOAD_CALLBACK),
+            'onload'            => OnloadCallback::get(),
             'data-sa-global'    => Setting::get(SettingName::SA_GLOBAL),
             'data-hostname'     => Setting::get(SettingName::HOSTNAME),
         ]);
diff --git a/src/SettingName.php b/src/SettingName.php
index fd35f98..b1cc789 100644
--- a/src/SettingName.php
+++ b/src/SettingName.php
@@ -45,6 +45,11 @@ class SettingName
      * @var string
      */
     const ONLOAD_CALLBACK = 'simpleanalytics_onload_callback';
+    /**
+     * Hash recorded only when an authorized user saves the callback.
+     * @var string
+     */
+    const ONLOAD_CALLBACK_HASH = 'simpleanalytics_onload_callback_hash';
     /**
      * @var string
      */
diff --git a/src/Settings/Blocks/Fields/Input.php b/src/Settings/Blocks/Fields/Input.php
index 031c341..93705ca 100644
--- a/src/Settings/Blocks/Fields/Input.php
+++ b/src/Settings/Blocks/Fields/Input.php
@@ -3,6 +3,8 @@
 namespace SimpleAnalytics\Settings\Blocks\Fields;
 
 use SimpleAnalytics\Setting;
+use SimpleAnalytics\SettingName;
+use SimpleAnalytics\Support\OnloadCallback;
 use SimpleAnalytics\Settings\Concerns\HasDocs;
 use SimpleAnalytics\Settings\Concerns\HasPlaceholder;
 use SimpleAnalytics\UI\LabelComponent;
@@ -29,6 +31,9 @@ class Input extends Field
 
     public function getValueSanitizer(): callable
     {
+        if ($this->getKey() === SettingName::ONLOAD_CALLBACK) {
+            return [OnloadCallback::class, 'sanitize'];
+        }
         return 'sanitize_text_field';
     }
 
diff --git a/src/Support/OnloadCallback.php b/src/Support/OnloadCallback.php
new file mode 100644
index 0000000..9118af0
--- /dev/null
+++ b/src/Support/OnloadCallback.php
@@ -0,0 +1,33 @@
+<?php
+
+namespace SimpleAnalytics\Support;
+
+use SimpleAnalytics\SettingName;
+
+class OnloadCallback
+{
+    public static function sanitize($value): string
+    {
+        if (! current_user_can('unfiltered_html')) {
+            $current = get_option(SettingName::ONLOAD_CALLBACK, '');
+            return is_string($current) ? $current : '';
+        }
+
+        $value = sanitize_text_field($value);
+        // This private option is not registered as an editable setting. It
+        // records that a user allowed to save JavaScript saved this exact code.
+        update_option(SettingName::ONLOAD_CALLBACK_HASH, hash_hmac('sha256', $value, wp_salt('auth')));
+        return $value;
+    }
+
+    public static function get(): ?string
+    {
+        $value = get_option(SettingName::ONLOAD_CALLBACK, '');
+        $hash = get_option(SettingName::ONLOAD_CALLBACK_HASH, '');
+
+        // Older releases accepted callback text without the capability check.
+        // Keep it inert until an authorized administrator saves it again.
+        if (! is_string($value) || $value === '' || ! is_string($hash)) return null;
+        return hash_equals($hash, hash_hmac('sha256', $value, wp_salt('auth'))) ? $value : null;
+    }
+}
diff --git a/tests/Browser/pluginSettings.spec.ts b/tests/Browser/pluginSettings.spec.ts
index 113bc71..c8ec555 100644
--- a/tests/Browser/pluginSettings.spec.ts
+++ b/tests/Browser/pluginSettings.spec.ts
@@ -190,6 +190,29 @@ test('adds a script with manually collect page views enabled', async ({ page, br
   await automaticGuest.context().close();
 });
 
+test('executes the saved onload callback when the analytics script loads', async ({ page, browser }) => {
+  await asAdmin(page);
+  await page.goto('/wp-admin/options-general.php?page=simpleanalytics&tab=advanced');
+  const callback = `document.documentElement.setAttribute('data-sa-callback', 'loaded "quoted"');`;
+  await page.locator('[name="simpleanalytics_onload_callback"]').fill(callback);
+  await saveSettings(page);
+
+  const context = await browser.newContext();
+  await context.route('https://scripts.simpleanalyticscdn.com/latest.js', route =>
+    route.fulfill({ contentType: 'application/javascript', body: '/* successful script load */' }));
+  const guest = await context.newPage();
+  try {
+    await guest.goto('/');
+    await expect(guest.locator('html')).toHaveAttribute('data-sa-callback', 'loaded "quoted"');
+    await expect(guest.locator(DEFAULT_SCRIPT_SELECTOR)).toHaveAttribute('onload', callback);
+    await expect(guest.locator(DEFAULT_SCRIPT_SELECTOR)).not.toHaveAttribute('data-onload');
+  } finally {
+    await page.locator('[name="simpleanalytics_onload_callback"]').fill('');
+    await saveSettings(page);
+    await context.close();
+  }
+});
+
 test('adds a script with overwrite domain name', async ({ page, browser }) => {
   await asAdmin(page);
   await page.goto('/wp-admin/options-general.php?page=simpleanalytics&tab=advanced');
diff --git a/tests/Regression/onload-permissions.php b/tests/Regression/onload-permissions.php
new file mode 100644
index 0000000..4e1fc63
--- /dev/null
+++ b/tests/Regression/onload-permissions.php
@@ -0,0 +1,38 @@
+<?php
+
+require __DIR__ . '/../Support/isolated-options.php';
+
+sa_with_isolated_options(static function () {
+    $key = SimpleAnalytics\SettingName::ONLOAD_CALLBACK;
+    $hashKey = SimpleAnalytics\SettingName::ONLOAD_CALLBACK_HASH;
+    $script = new SimpleAnalytics\Scripts\AnalyticsScript();
+
+    delete_option($hashKey);
+    update_option($key, 'legacyCallback()');
+    sa_assert(! isset($script->attributes()['onload']), 'Unverified legacy text must remain inert.');
+
+    $admin = get_user_by('login', 'admin');
+    sa_assert($admin !== false, 'The wp-env admin user must exist.');
+    wp_set_current_user($admin->ID);
+    $field = new SimpleAnalytics\Settings\Blocks\Fields\Input($key, 'Onload Callback');
+    register_setting('simpleanalytics-advanced', $key, ['sanitize_callback' => $field->getValueSanitizer()]);
+    update_option($key, 'authorizedCallback()');
+    sa_assert(($script->attributes()['onload'] ?? null) === 'authorizedCallback()', 'Authorized saves must enable the callback.');
+
+    // Simulate a multisite administrator (or DISALLOW_UNFILTERED_HTML) through
+    // WordPress's real capability mapping, without changing any user records.
+    $deny = static function ($caps, $cap) { return $cap === 'unfiltered_html' ? ['do_not_allow'] : $caps; };
+    add_filter('map_meta_cap', $deny, 10, 2);
+    $hash = get_option($hashKey);
+    update_option($key, 'unauthorizedCallback()');
+    sa_assert(get_option($key) === 'authorizedCallback()', 'Users without unfiltered_html must not replace the callback.');
+    sa_assert(get_option($hashKey) === $hash, 'Unauthorized saves must not change the recorded hash.');
+    remove_filter('map_meta_cap', $deny, 10);
+
+    remove_filter('sanitize_option_' . $key, $field->getValueSanitizer());
+    update_option($key, 'changedOutsideSettings()');
+    sa_assert(! isset($script->attributes()['onload']), 'Code that differs from the verified value must remain inert.');
+    register_setting('simpleanalytics-advanced', $key, ['sanitize_callback' => $field->getValueSanitizer()]);
+    update_option($key, '');
+    sa_assert(! isset($script->attributes()['onload']), 'Clearing the callback must remove the handler.');
+});