"Kate Moss is my friend ... I think it's like everybody is being bad to her," the 35-year-old supermodel told a news conference Sunday in the Colombian capital where she was judging a modeling competition.
"It's not the first time it has happened in the world ... it's really like a vendetta," said Campbell, who has said that she nearly self-destructed from her use of cocaine.
Pictures published in a British tabloid appeared to show Moss using cocaine. Moss, 31, issued a statement last week apologizing to "all the people I have let down."
"I take full responsibility for my actions," she said. Her statement stopped short of specifying whether she had used cocaine.
Meanwhile, the modeling agency in London that represents Moss said it would support her in the face of the allegations.
"I feel compelled to speak out in defense of Kate Moss following recent days of speculation and, in many cases, ill-informed and inaccurate stories," Sarah Doukas, director of the Storm agency, said in a statement Sunday.
"I know Kate, and I have represented her for the last 18 years. Over this time she has consistently demonstrated herself to be a professional and exceptional model and a loyal, special and dear friend to me and countless others," Doukas said.
Campbell said it was wrong to blame the modeling industry for drug abuse and eating disorders among young women.
She agreed to take part in a program run by the United Nations Office on Drugs and Crime in Colombia to raise awareness about human trafficking.
Campbell said she wanted to help in "providing a safe environment for models and preventing exploitation of human beings."
More than a dozen Colombian models from the Elite modeling agency recently attended a U.N. course in Bogota on how to spread the message about the risks of being lured abroad and sexually exploited.
Colombian police estimate that up to 50,000 Colombians, including many underage girls and boys, have been forced into prostitution, mainly in Japan, Spain and Holland.