Cherie Madge, 42, was found guilty of posting drugs to her internet "soul mate" Shaun Bowmer in Penrith, Cumbria.
She continued over a period of months despite pleas from Mr Bowmer's partner to stop. He died in February 2016.
Jailing her for four-and-a-half years at Carlisle Crown Court, Judge Peter Davies said Mr Bowmer "may be alive today" but for Madge.
The court heard the pair met in an online chat room "perhaps as along as 10 years ago".
Though they spoke to each other over the internet and phone, they never physically met.
Jurors were told Madge sent Mr Bowmer patient-returned drugs she sourced from her place of work between August 2015 and February 2016.
High levels of tramadol and morphine were found in Mr Bowmer's system after his death.
Madge, of Harlow in Essex, admitted sending Mr Bowmer tramadol, but denied illegally supplying him with four other controlled drugs - morphine-based Class A Zomorph and Sevredol and Class C diazepam and lorazepam.
Jurors found her guilty of all offences.
Judge Davies said Mr Bowmer's death was a "seriously aggravating feature" of Madge's offending.
He told her: "It is not an exaggeration to say but for you, he may be alive today."
A pharmacy assistant who illegally sent medication to an addict who then died from "drugs toxicity" has been jailed.