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Using Drupal behaviors and jQuery once, the code would be

    (function($) {
       Drupal.behaviors.customToggler = {
         attach: function(context, settings){
            $(".toggler", context).once('custom-toggler').click(function(){
              $(this).next().slideToggle("slow");
              return false;
            }).next().hide();
          };
        };
    })(jQuery);

This allows the script to work for elements added after the page loads.