There's actually a very valid reason for this which originates in the foundations of how the pharmacy indurstry operates.

Manufacturers maintain their pricing for the prescription version of medications who have gone over the counter, because at those dosage levels, it becomes a prescription item which is reimbursable by insurance. (More than 90% of prescriptions dispensed in the U.S. are primarily insurance reimbursed). Pharmacies still pay considerably higher prices for these medications from their wholesalers and manufacturers, even though the OTC versions are sold much less expensively. Even medications like Ibuprofen, Naproxyn, Tagamet, Zantac, and a host of others are still available in presription-only doses. And while exceptions are the only rule, this is relavent to all but a few exceptions, of which none come to mind off-hand.

So, why don't doctors just tell their patients to take the equivalent amount of an OTC version to equal a prescription dose? There are several reasons. First of all, for patients with insurance, it makes it non-prescription which is covered (in all but the most rare cases and third-party plans). Second, when it comes to prescribing, the rate of compliance, which is the degree to which patients take their medications as prescribed, is HIGHLY affected by the number of times, number of doses, and the manner in which customers much take their prescribed meds. This is why insurers will still pay the premiums for prescription doses, as there is some valid medical basis that there is a tangible potential benefit to the patient.

This concept is also demonstrated in medications that may change delivery mechanisms, like Oxycodone to OxyContin, and Prozac to Prozac Weekly. Regarding Oxycodone, it has been available for decades. However, because OxyContin was intended to provide a sustained level of pain management around the clock with only 2 or 3 dosage intervals per day, and combined with its theoreticaly low abuse potential, it commanded its relatively exhorbitant price, in some cases, more than 1000%, yet even insurers who require generic usage will cover OxyContin based on its therapeutic merits. Prozac, which went off patent, is chemically identical to the active ingredient in Prozac Weekly. Insurance plans that cover Prozac were rejoicing when its patent expired, and insurers rejoiced as they would be able to cover far less expensive generic versions of the anti-depressant. However, because compliance can be so much better for patients who take Prozac Weekly, (the problem with a lot of anti-depressants is that people stop taking them when they perceive themselves as feeling better, which permits depression to recur) insurers typically cover the hugely expensive new version in recognition of its improved compliance.

Yes, in many cases, it is a matter of pharmaceutical companies maintaining the highest possible prices, and despite the fact that the end result is never seen directly by most patients (as 90%+ of prescriptions are insured), of course the costs are eventually paid by those who pay insuranc premiums etc... that's a WHOLE different discussion for another thread, indeed probably another board altogether. Nevertheless, when such a pricing discrepancy exists for what should seem to be a different dosage of an identical medication, there is invariable some therapeutic basis for justification.