Petite, blonde and bespectacled, 78-year-old Melissa Ann Shepard may have seemed an unlikely femme fatale, but that was precisely what made her so dangerous. This week, Shepard was sentenced to three and a half years in prison for drugging her elderly husband on their honeymoon: the latest in a string of such incidents that has earned her the title of Canada's "Black Widow".

In September 2012, as the newlyweds travelled by ferry from Nova Scotia to Newfoundland, 76-year-old Fred Weeks fell ill after drinking coffee laced with tranquillisers. What he didn't know was that his new wife had made a habit of outliving her husbands. In 1992, she was convicted of manslaughter for drugging her then-husband, Gordon Stewart, and running him over – twice – near Halifax. She served two years of a six-year sentence.

Following her release, Shepard moved south to Florida, where she met her third husband, Robert Friedrich, at a Christian retreat. They married in 2000, but Friedrich died two years later of a heart attack, having suffered a series of fainting spells that were never explained.

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In 2005, Shepard was sentenced to five years in prison for stealing $20,000 from her boyfriend Alex Strategos, now 81. Strategos was hospitalised a number of times during their brief relationship; a blood test at the time of the trial showed he had traces of the sedative benzodiazepine in his system.

Last August, free once more, Shepard reportedly moved into the same retirement complex as Mr Weeks in the small Nova Scotia town of New Glasgow. After a whirlwind romance, the two were married on 25 September 2012 – though the marriage would later be declared invalid, after false information was found on the marriage certificate.

The day after the wedding, the couple set off across the Cabot Strait from Cape Breton. By the time they reached Channel-Port-aux-Basques, Newfoundland, Mr Weeks was unable to walk. The couple returned to Nova Scotia to stay in a B&B, where he fell out of bed and was rushed to hospital. Shepard then lied to medical staff, claiming her new husband had prostate problems, dementia and chronic obstructive pulmonary disorder. She also insisted that Mr Weeks – a father of six – had no other family.

When Shepard was finally arrested and charged in October, police found a handful of prescription medications among her belongings. They also seized notes written on hotel stationery with the words "lawyer", "power of attorney" and "will".

Shepard's lawyer, Allan Nicholson, however, claimed she had not intended to harm her husband, but had miscalculated the dosage of tranquillisers. As for the notes, Mr Nicholson said: "She was an ex-secretary. She was looking after business."

An attempted murder charge was dropped because the prosecution could not prove Shepard's intent to kill. Instead, she pleaded guilty on Monday to administering a noxious substance, and failing to provide the necessities of life. Her sentence will be reduced to two years and nine months to reflect time already served, but Judge Joseph Kennedy warned: "People who have contact with this lady should be careful. Do not allow yourself to be victimised."

Meanwhile, Mr Weeks told the Cape Breton Post that he still suffers lapses of memory – but, remarkably, harbours no ill will. "I got no bitter feelings or anything like that," he said. "A lot of people think I'm right bitter because I almost died. Well, almost doesn't count. Not to me, anyway."