A police spokesman said: "The woman was arrested on suspicion of causing or allowing serious physical harm to a child. She has been released on police bail until August 13. Inquiries are ongoing."
The woman, who has not been identified, gave the girl the strong prescription painkiller and she went back to sleep.
However, the next morning, she couldn't wake the girl and had to call an ambulance, the Daily Mail reports.
Doctors initially feared she would not make it through the night and warned she could suffer brain damage.
But police said she was now in a 'stable condition' at the Royal Manchester Children's Hospital.
A police officer was put on guard outside the mid-terrace house in Breightmet Bolton, and the girl's 31-year-old mother was arrested.
According to the paper, the woman described her actions as a 'stupid mistake', and added added: "(My daughter) woke up around midnight with severe toothache, and was screaming in absolute agony.
"I just wanted to help her. I did not want to see her in pain.
"Our lives changed so fast. This was a stupid mistake, I just wanted to make her better.
"She was taken to hospital and the doctors were telling me she might not make it through the night, and if she did she could have brain damage and would not be the same as she was before.
"I felt like my soul had been ripped out. It did not feel real, I was in shock.
"I feel guilty beyond words. I just hope she can forgive me and I have got to forgive myself now.
"I was arrested and interviewed by the police. I have been open and honest with them about what happened from the word go."
She added: "She has got good angels looking after her. She wants me to buy her a Blackberry now, I think she deserves to be spoiled for a bit."