Monteith passed away from a heroin and alcohol overdose at the age of 31 in a hotel room in Vancouver, Canada in July (13), and Nicks was so stunned by the tragedy she was physically ill.
The singer, who battled a cocaine addiction for 10 years at the height of the Don't Stop hitmakers' success, had grown close to the cast of the U.S. Tv show after visiting the set for an episode themed around Fleetwood Mac's landmark album Rumours.
She was so shocked after Monteith's death that she felt compelled to pen a note to his fellow screen stars offering her support.
Nicks tells British magazine Stella, "I said, 'Forgive me if I'm stepping into your personal space. I lost my godson to an overdose last year - he died at a fraternity party - so I do understand how shocking that is.'
"When what happened to Cory happened, I was just sick for days. Honestly, just sick. (Drugs are) just so prevalent. It's everywhere. It's worse than when we were young."