Lohan wrote on her Twitter page Friday: "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. Substance abuse is a disease, which unfortunately doesn't go away overnight. I am working hard to overcome it and am taking positive steps."
Under the terms of her probation, the 24-year-old could be sent back to jail for 30 days. A hearing would be conducted before Lohan could be returned to jail.
The Los Angeles County District Attorney's Office said it had not received word of a failed test. A call to Lohan's attorney was not returned Friday.
Lohan spent three weeks at a drug rehabilitation program last month after serving 13 days in jail for violating her probation in two DUI cases.
With her probation, Lohan is subject to random drug and alcohol testing several times a week.
The Associated Press contributed to this report.