Unlike most other medications that we give to children, the dose of Concerta that is effective for treating ADHD symptoms is not determined just by weight or age. While we dose antibiotics and Tylenol based on how many pounds a child weighs, Concerta's effects are not dependent on body mass. Rather, Concerta works by affecting how the neurotransmitters (the chemicals that nerves secrete to transmit impulses) work. The amounts and effectiveness of these transmitters vary from person to person and don't necessarily correspond to weight or age.
The smallest size Concerta pill is 18 mg, and it also comes in 36 and 54 mg pills. Some children are well treated with 18 or 36 mg, though others may need more. The maximum recommended dose is actually 72 mg, using combinations of the various pills. Thus your daughter is at the upper limit, but not over it.
So how do you tell if a child is getting too much stimulant medication? Diminished appetite and difficulty sleeping are the most common signs. Children can also appear "dull" or oversedated if the dose is too high. Very rarely, children can have mood disturbances or psychotic reactions with hallucinations if they are overdosed. If your daughter is functioning well, with control of her ADHD symptoms without any of these side effects, then it is unlikely that she is overmedicated.