The single mother who is about to marry Norway's Crown Prince Haakon spoke about her rebellious days in an emotional press conference on Wednesday.
Miss Hoiby, mother of four-year-old Marius, has been at the centre of media attention since the crown prince announced he was dating her, 15 months ago.
Drugs condemned
"We overstepped the limits," said a tearful Miss Hoiby. "It was a costly experience for me, that I took a long time to get over."
About half of Norway's first-born children are born to single mothers, and polls show that most Norwegians do not care that Miss Hoiby is a single mother or that the couple chose to live together in an Oslo flat before marrying.
This is despite the fact that when Crown Prince Haakon succeeds King Harald, he will become head of the state Lutheran church, which opposes cohabitation outside matrimony.
"I would like to take this opportunity to say that I condemn drugs," Miss Hoiby said at the press conference, but did not say whether she had taken them herself.
"I cannot make these choices again, even though I would wish I could," she said.
First royal wedding in years
At the end of her remarks, the future crown princess asked the media to leave her past alone.
"I hope that I can now avoid talking more about my past, and that the press will respect this wish."
The guests are to include Britain's Crown Prince Charles and Prince Edward, Sweden's King Carl XVI Gustaf and Denmark's Queen Margrethe II and their families, King Albert II of Belgium, Prince Albert of Monaco, and Spain's Crown Prince Felipe.
The wedding ceremony itself will be held in the Oslo Cathedral on Saturday.
Parents supportive
Crown Prince Haakon said his parents have been very supportive of his relationship with Miss Hoiby.
"What we two found together was so strong that I could not let it go," said the crown prince, who was sitting beside Miss Hoiby during the press conference.
It had taken his father, King Harald, nine years to win the permission of his father, King Olav V, to marry the commoner, Sonja Haraldsen.
Crown Prince Haakon said his parents approved of his choice and had expressed to him that "nine years might have been a bit long to wait".
Miss Hoiby's son, Marius, will never be able to become king under Norway's constitution.
But the young couple have said they plan to make him part of a big royal family.