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It’s important to know whether you are comparing the brand name product to the brand name product of each strength, rather than brand of one strength to generic of the other, and also, if using generics, comparing generic to generic of the same generic manufacturer! Concerta® (brand) is one of the rare medications where the FDA issued a warning a few years ago (but did not order a recall) that the generic “Concerta” of 2 of the 3 generic manufacturers did NOT meet the bioequivalency specifications of the brand name!! Even insurance companies agreed to cover the brand, in light of this, rather than have to deal with complaints of ineffective or inconsistent clinical effectiveness from differences among the generic products.
The immediate release portion of Concerta has been stated to be 22% of the total dose, and is in the “overcoat” of the tablet, the surface.
One possible consideration to explain your differences in effectiveness of the 2 18mg tablets vs. the 1 36mg tablet, “might” be that the surface area of 2 tablets is greater than the surface area of one tablet, thus, the amount of immediate release coated surface area exposed to immediate dissolving in the stomach “might” be why it is dissolved sooner, and thus absorbed more quickly than the same total amount of immediate release medication on the surface of the single 36mg tablet. Just a thought ;—)
Your question seems to be addressed by the manufacturer (of the brand name product) in the second paragraph below, where they describe the equivalence of 2 27mg tablets and 3 18mg tablets.
“5.2. Pharmacokinetic properties Absorption
Methylphenidate is readily absorbed. Following oral administration of CONCERTA to adults, the drug overcoat dissolves and plasma methylphenidate concentrations increase rapidly reaching an initial maximum at about 1 to 2 hours. The methylphenidate contained in two internal drug layers is gradually released over the next few hours. Peak plasma concentrations are achieved at about 6 to 8 hours after which plasma levels of methylphenidate gradually decrease. CONCERTA once daily minimises the fluctuations between peak and trough concentrations associated with immediate-release methylphenidate three times daily. The extent of absorption of CONCERTA once daily is generally comparable to conventional immediate release preparations given three times daily.
“Pharmacokinetic equivalence has been demonstrated for two 27-mg CONCERTA tablets with three 18-mg CONCERTA tablets. The mean values of the treatment ratio (2 x 27 mg fasted/3 x 18 mg fasted) of the log- transformed pharmacokinetic values for Cmax, Tmax and AUCinf were 101.1%, 104.3% and 100.3% respectively. The 90% CIs for the treatment ratios were within the pre-specified 80% – 125% range.”
Some people feel the immediate release portion of the tablet is insufficient to give an initial, immediate, adequate clinical response. This is sometimes treated by taking a small dose of an immediate release methylphenidate tablet (Ritalin or generic) with the Concerta, which gives a stronger immediate effect, beginning 20-30 minutes after administration, while the extended release Concerta is ‘catching up.”
Hope this is helpful.