diff --git a/filter/tex/lib.php b/filter/tex/lib.php
index cb9ad37423e..b95591bc091 100644
--- a/filter/tex/lib.php
+++ b/filter/tex/lib.php
@@ -102,7 +102,9 @@ function filter_tex_sanitize_formula(string $texexp): string {
     // First, mangle all denied words.
     $texexp = preg_replace_callback($denylist,
         function($matches) {
-            return 'forbiddenkeyword_' . $matches[0];
+            // Remove backslashes to make commands impotent.
+            $noslashes = str_replace('\\', '', $matches[0]);
+            return 'forbiddenkeyword_' . $noslashes;
         },
         $texexp
     );
diff --git a/filter/tex/tests/lib_test.php b/filter/tex/tests/lib_test.php
index e5bfe29cf37..a718f9c793f 100644
--- a/filter/tex/tests/lib_test.php
+++ b/filter/tex/tests/lib_test.php
@@ -49,7 +49,7 @@ final class lib_test extends advanced_testcase {
             ['x\ =\ \frac{\sqrt{144}}{2}\ \times\ (y\ +\ 12)', 'x\ =\ \frac{\sqrt{144}}{2}\ \times\ (y\ +\ 12)'],
             ['\usepackage[latin1]{inputenc}', '\usepackage[latin1]{inputenc}'],
             ['\newcommand{\A}{\verbatiminput}', '\newforbiddenkeyword_command{\A}{\verbatimforbiddenkeyword_input}'],
-            ['\pdffiledump offset 0 length', 'forbiddenkeyword_\pdffiledump offset 0 length'],
+            ['\pdffiledump offset 0 length', 'forbiddenkeyword_pdffiledump offset 0 length'],
         ];
     }