Howard K. Stern injected Anna Nicole Smith with drugs — and kept her supplied with middle-of-the-night pill runs, charges her personal assistant in a world-exclusive ENQUIRER interview.

The out-of-control star took 20 to 30 powerful pills at a time — using a variety of fake names and physicians to make sure she would never run low, reveals Nathan Collins — who witnessed the blonde beauty's descent into hell.

What's more, her constant companion Stern issued strict orders not to call 911 if Anna overdosed, Nathan told The ENQUIRER.

A year before she died, Anna and Stern got into a heated argument after the model had her doctor double the strength of her anti-depressant medication Lexapro — and Stern argued that the dose could be fatal.

"But Stern backed off because he was nothing but an enabler to Anna," Collins declared.

Nathan's full interview is in the new issue of The ENQUIRER.

Click here to watch video of part of Collins interview