After Casey died at age 20, her mum granted her final wish in the most heartbreaking way.

It was the obituary mum Michelle Schwartzmier never wanted to write, but to keep a promise to her daughter she did.

Michelle’s daughter Casey Schwartzmier was just 20 years old when she dies of a heroin overdose on January 15.

The Pennsylvania woman was addicted to the drug – a grave problem her mother knew about and was trying to help her overcome, local broadcaster Channel 11 reports.

Michelle revealed Casey had been organised to travel to a clinic in Calfornia for rehabilitation on the day after her overdose.

Despite her efforts to recover, Casey had told her mum that if something ever happened to her, she wanted her to write an honest obituary in the hopes it would deter others from following the same path.

Michelle said writing her daughter's obituary was the "hardest thing I've ever done in my life".

"I wanted to honour her memory," she told the news station.

"I hope I hit the right words because I feel like Casey was helping me write that."

In it, Michelle described her daughter as a "feisty and outspoken" girl with a "loving beautiful heart".

Michelle said her daughter did not want to be defined only by her mistakes, and hoped that sharing the story of a child from the suburbs gripped by the drug could "help break the stigma about addicts".

"Casey believed that hiding her cause of death would help no one, but if her story could help just one addict push even harder for another day of sobriety, encourage an active user to choose recovery or shine a light on this horrible epidemic, then it would be worth coming out of the shadows," Michelle wrote.

Casey had donated her organs, an act her mum believed was an attempt at redemption.

"Casey believed strongly in second chances, maybe because she craved another chance for herself and other addicts, so she donated her life-saving organs to give someone else, a second chance at life," the obituary reads.

Michelle said she hoped people reading would "think twice" before judging an addict.

"Addiction doesn't discriminate, it will take hold and destroy anyone in its path, including the families and people who love them," she wrote.

"Addiction hides in the faces of everyday people all around us. Casey isn't just another statistic or just ‘another one gone too soon,’ she was a great heart with a bright future and a gift that the world lost and can never be replaced."

You can read Casey's full obituary at News WPXI.

If you need help or are struggling to cope or would like to talk to someone who can help call Lifeline on 13 11 14. You can visit this website for more information about drug support services near you.