In exchange for her plea, the state has agreed to drop five other drug-related charges against the 42-year-old Wasilla woman, who is the other grandmother to Gov. Sarah Palin's grandson Tripp.
The plea agreement was made public at a court hearing in Palmer.
The deal could include a reduced sentence, Palmer assistant district attorney Alison Collins said in court Thursday.
Butler said he is asking for a reduced sentence because of the small amount of drugs involved.
According to police affidavits, informants bought 10 OxyContin pills from Johnston on three separate occasions in October and November last year. She was arrested in December and released on bail a few hours later.
Johnston is the mother of Levi Johnston, who recently fathered a child with Bristol Palin, Gov. Sarah Palin's daughter. The couple say they broke up shortly after their son was born. Sherry Johnston and her daughter, Mercedes, recently accompanied Levi Johnston on a tour of talk shows such as "Larry King Live" and "The Tyra Banks Show." Johnston had to get prosecutors' permission to leave the state.
Mercedes, Levi and another young man were in the courtroom Thursday to support Johnston.
Before Johnston changes her plea at a hearing set for the end of July, Butler said he hopes to work out a deal with the state Department of Corrections to minimize the amount of time she spends actually locked up.
Pleading guilty to a felony means an automatic ticket to jail, Butler said, and Johnston has medical issues that make jail time a problem. He hopes to get the state Department of Corrections to agree that Johnston can serve her sentence at home with an ankle monitor instead.
Johnston said she is being treated for chronic pain by a device installed under her skin in her side that delivers very small amounts of medication directly into her spinal fluid. Another small machine, an "epidural stimulator," is installed on her lower back to block pain.
Butler had a letter from Johnston's doctor, Leon Chandler at A.A. Specialty Health Center in Anchorage. It outlined her condition and said a limited number of medical professionals are trained to operate the equipment being used to treat Johnston.
"I want to be sure her needs are met. I know they can be met at home on an ankle monitor because that will give her time to go to her doctor," Butler said.
Johnston said she visits the doctor about twice each week.
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