Lyndsay Jones, 36, of no fixed address, pleaded guilty to the gross negligence manslaughter of Philip Makinson, who died at a flat in Bolton in February.
Police said it was "not a mercy killing" and Jones, a heroin addict, was "not motivated by altruism but by greed".
She was jailed for four and a half years at Manchester Crown Court.
Mr Makinson was suffering from "crippling depression and anxiety" following the death of his sister-in-law and mother in 2012, the collapse of his business and the break-up of his relationship, police said.
He had previously made two attempts to kill himself and enlisted the help of acquaintance Jones.
On 1 January last year, he asked Jones to buy him some heroin and prepare an injection which would bring about a fatal overdose , said police.
However, officers said, in an attempt to keep some of the heroin for herself, Jones did not give Mr Makinson enough to kill him and he was found by friends having suffered a non-fatal overdose.
A month later, Mr Makinson again enlisted Jones' help to bring about his death, and on the second occasion the dose was fatal.
Jones told her then-partner about what she had done, and he recorded the conversations on his mobile phone. When the pair split up last year he told police.
Det Ch Insp Richard Jackson said: "Mr Makinson needed psychological help but was instead injected with heroin and left alone to die by a woman whose primary motivation was to obtain drugs for her own use.
"A man came to her in desperation and instead of providing a helping hand, she provided the killer blow."
A woman who gave a lethal heroin injection to a man who wanted to end his own life has been jailed.