Many people, even those who know a lot of chemistry, think that a pill with 80 mg of an active drug compound is the same product when it comes from a generic provider. Governments in Australia and USA also seem to think this as they subsidize only the cheaper generic version of drugs when prescribed to low income earners.
As I learned, these are not the same ingredients as they often differ by crystal form. Many compounds can be present in more than one crystal polymorph, or they can be present in crystals of varied size and size distribution. This is called crystal form and a consistent crystal form is achieved by adherence to a repeatable method of preparation. Pfizer determine optimal dose of a drug compound on it being prepared with a consistent crystal form. When they develop a drug through clinical trials with patients they optimize its dosage to perhaps 25 mg, 70 mg or 110 mg by careful monitoring of the patients taking part. When taken orally, crystal form can greatly influence bioavailability of a drug. Some reach their target better when absorbed through the stomach, others the intestine. Therefore the dose chosen to deliver the optimal drug availability to the patient is based upon Pfizer's consistent crystal form.
A less uniform crystal form in the generic version of the drug disturbs this carefully optimized dose for the patient. For example, the drug may be intestinally absorbed with some being 'lost' in the stomach - 80 mg may be actually delivering 60 mg to the intestine when in the consistent crystal form prepared by Pfizer. In the generic version, the crystal form may dissolve more readily in the stomach and only 20 mg survives to be absorbed in the intestine. The opposite can also be true - a drug made with a crystal form that is easy for the stomach to dissolve and only a small amount is 'lost' to the intestine where absorption is less effective, while the generic version takes more undissolved drug with it to the intestine.
A random crystal form disturbs and even ignores the intended dose for the patient.Generic drugs are therefore often less effective than the brand name version. You might want to think about this the next time you or a loved one goes to the pharmacy.