Lindsay Lohan Lindsay Lohan

Just a few weeks after leaving rehab, Lindsay Lohan has admitted that she failed a court-ordered drug test.

"Regrettably, I did in fact fail my most recent drug test and if I am asked, I am prepared to appear before judge Fox next week as a result," Lohan tweeted late Friday.

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On July 6, Lohan was sentenced to 90 days in jail to be followed by 90 days of inpatient rehab. She spent just 13 days in jail because of overcrowding, and stayed only 23 days in a substance abuse program. Lohan is currently undergoing random drug testing twice a week and attending  multiple behavioral therapy and psychotherapy sessions each week.

"Substance abuse is a disease, which unfortunately doesn't go away over night," Lohan also wrote on Twitter. "I am working hard to overcome it and am taking positive steps forward every day. I am testing every single day and doing what I must do to prevent any mishaps in the future."

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The 24-year-old starlet could face up to another 30 days in jail for violating her probation.

"This was certainly a setback for me but I am taking responsibility for my actions and I'm prepared to face the consequences," she said.