Nav-pills open to the left

I'm following the following code:

<ul class="nav nav-pills">
  ...
  <li role="presentation" class="dropdown">
    <a class="dropdown-toggle" data-toggle="dropdown" href="#" role="button" aria-haspopup="true" aria-expanded="false">
      Dropdown <span class="caret"></span>
    </a>
    <ul class="dropdown-menu">
      ...
    </ul>
  </li>
  ...
</ul>

But I would like the dropdown to open to the left. I tried the class pull-left but without success.

Any help?