cited. The petition cited a series of messages posted on the online social network Twitter.com as evidence that Johnston may have substance abuse issues, based on statements he has made about seeking weed. Johnston's response said he does not have a Twitter account and has made no such statements about the use of marijuana. But Palin's harshest attack was on Levi's mother, Sherry Johnston, who pleaded guilty to a felony drug charge earlier this year. Palin asked that the paternal grandmother be barred from having any unsupervised visitation with Tripp. Her petition said she may re-offend, keep medications around the house which could be accessible to the baby, have illegal drug users come to the house -LRB- or she could take the baby in her car during a drug sale -RRB-. Palin also said that since Sherry Johnston could fall asleep or be too drowsy to appropriately monitor the baby. She is also a chronic pain patient and undergoes daily, regular and sustained narcotic infusions, Palin's petition said. Johnston's response said his mother's chronic pain condition is currently being managed in coordination with the Department of Corrections. Sherry Johnston will not be left to take care of the minor child alone, but will simply be there to assist to the best of her ability, Johnston's filing said. She was released from an Alaska prison last week to serve the remainder of her