[Question] Conversion From Ritalin

Question:

Can you recommend a conversion from Ritalin 10mg tid to Focalin?

Answer:

Focalin, Novartis’ brand of dexmethylphenidate (there is a generic product available from Teva Pharmaceuticals) is the dextrorotatory form of methylphenidate (Ritalin, also from Novaris, is the best known brand of methylphenidate) which is used for treatment of attention deficit disorder. If anybody wants a really good discussion of stereochemistry and how it affects drug therapy, here’s a link to a paper that appeared in the Primary Care Companion to the Journal of Clinical Psychiatry but I’ll try to offer an explanation anyway.

Lots of chemical compounds can rotate a beam of polarized light, and lots of compounds exist in two forms, mirror images of each other. These compounds will have the same number of atoms, the same molecular weight, the same formula, but (for want of a better example), like your two hands, they don’t fit in the same glove. The molecular structure that twists a beam of polarized light to the left is called the levorotatory form, and the chemical structure that twists the beam to the right is called dextrorotatory. A mixture of the two forms is called racemic, and because it has equal amounts of the dextro and levo forms of the compound, a racemic mixture won’t bend a beam of polarized light at all.

As a rule, only the dextro form of a compound has any medicinal effect — the levo is inactive. There are exceptions, but no important ones. So, it’s common enough to separate the two compounds and use only the active form. When you see a drug make that begins with dex or dextro, it’s almost certainly the dextrorotatory half of a mixture of dextro and levo molecules — and the dose is just half of the dose that would be used for the mixed form. So, from the dexmethylphenidate package insert, the dosing instructions are:

For patients currently using methylphenidate, the recommended starting dose of dexmethylphenidate IR or ER is half the dose of racemic methylphenidate. Patients currently using dexmethylphenidate IR may be switched to the same daily dose of dexmethylphenidate ER. The maximum recommended dose for children and adults is 20 mg/day (10 mg twice-daily IR).