Both Ritalin and Risperdal (and any other ADHD medication for that matter) reduces the re-uptake of different neurotransmitters in the brain. Which medication is best suited to your son depends on the type of ADHD he has (Inattentive, Hyperactive or Combined type.) Ritalin works or dopamine and noradrenaline, Risperdal works on dopamine (among others).
ADHD is caused by the lowered levels of dopamine and/or noradrenaline. Dopamin helps us to concentrate while noradrenalin reduces impulsive behaviours.
Very basically put if your son is the inattentive type, he would benefit more from a medication that increases dopamie levels; if he is more hyperactive type he would benefit more from medications that increase noradrenaline levels. As there are a few medications out there which can achieve this, the one best suited to your son ultimately depends on how he responds to them. It can be a trial and error basis.
Please read the article at http://www.health24.com/medical/Condition_centres/777-792-4382-4384,60875.asp for further information.
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