The reason that is often stated is that a generic drug is cheaper to purchase, as the company making the drug doesn't have to recoup all of the money that went into developing and marketing the original drug (which runs into the millions of dollars, to bring a new drug to market). However, a generic drug must contain the active drug, but the "inactive" part of the medicine is not dictated. So, unfortunately, the inactive part of the medicine may interfere with the absorption of the active drug. To be accepted for sale, a generic drug must only be within a certain percentage of efficacy of the brand name drug (I think it's around 15 percent or so).
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Prescription drugs|Prescription medicines