The toddler was with his mother and grandmother at a McDonald's in southeast London when a stranger gave him the bottle Thursday afternoon, Sky News reported.
The baby was in stable condition at a London hospital,
Police said they believed he was the only victim.
The bottle was recovered by police and its contents are being tested.
A 53-year-old woman was arrested Friday morning in Deptford.
Celebrity Families of 2014 [PHOTOS]