The majority of the controlled trial data with guanfacine in ADHD is with the brand name only GXR (Intuniv). The older, immediate-release guanfacine is available, in identical strengths, for a much cheaper price—about $0.80 per tablet versus over $8 per tablet for Intuniv. Seems like a no-brainer to substitute the generic, right? Well, the manufacturer of Intuniv warns against substituting the cheaper, generic, immediate-release guanfacine for its product. The company bases this on the differing pharmacokinetic properties. From data in adults, the extended-release formulation of guanfacine is reported to have a peak serum level that is about 60% lower and occurs three hours later than that with immediate-release. This is not an unusual pharmacokinetic difference when comparing an extended-release to an immediate-release formulation and, oftentimes, this relates to lower rates of adverse effects seen with an extended-release formulation. Interestingly, rates of somnolence in patients taking GXR as monotherapy compared to patients taking immediate-release guanfacine appear similar when comparing reported frequencies in the package inserts (not head-to-head studies unfortunately, but the best we have for comparison).
Price and Pharmacokinetics
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