Most people think that generic drugs always work, that all Tier I drugs of an HMO are pharmacologically equivalent,and that generic drugs are cheaper. Only the last statement is true, and the pharmacies also love generics because they have higher profit margins. (Brand name $80, charge to you $100, profit$20; generic $20, price to you $50, profit $30.) However, the generics do not always work as well as brand names, because there can be different "inert" compounds added to the parent drug to make the tablet. The FDA requires that the blood levels of the generic be the same as the brand name (+/- 10%), but only the brand name drug has been tested for clinical efficiency. There is no way of comparing tissue levels of the brand name drug to the generic, and this is especially important with drugs that act on the central nervous system (tranquilizers, sleeping pills, pain pills) after crossing the blood-brain barrier. Again, no generic is tested for clinical efficiency against the brand name drug.
The blood-brain barrier is nature's way of protecting the brain from noxious compounds that are dissolved in your blood. A drug has to cross the blood-brain barrier (through capillary walls) to reach the brain and achieve its effect. Whether a particular drug can do this is usually only determined by trial and error. For instance, penicillin readily crosses the blood-brain barrier, and is therefore extremely active in treating meningitis. Keflex, a cephalosporin, cannot cross the blood-brain barrier, and is therefore useless in treating meningitis, although it generally can treat any soft tissue infection or lung infection that penicillin does.
Within a particular class of drugs (e.g. beta-blockers) if a drug is lipophilic, this affects the ability of the brain to absorb the drug once it gets there. Marijuana is extremely lipophilic, so it is released very slowly from fatty tissue in the brain, and your urine can test positive for at least two weeks after inhaling it. There can also be subtle differences within a class of drugs: Coreg is the only beta-blocker shown to reduce microalbinuria in diabetic patients.
When it comes to other groups of drugs, there is wide human variation in responsiveness, since we are all biochemically different. Claritin and Zyrtec are over-the-counter (OTC) anti-histamines, and sometimes one will work on a patient's allergies, and sometimes the other, since they are chemically very different. Allegra is a prescription antihistamine, which works better in many patients, but unlike Medicare Part D, most HMO's ask me to certify that both OTC drugs did not work before they will pay for Allegra. The same problem arises is proton-pump inhibitors (PPI's) for acid reflux. The HMO's want me to try the patient on OTC Prilosec before paying for Prevacid, Aciphex or Nexium. Furthermore, if I want to prescribe Nexium at a double dose to suppress acid symptoms, which some patients need, I generally have to send them to a GI doctor to have the payment okayed. Remember, if you are forced to buy an OTC drug, your co-pay is 100%, and the cost to your HMO is $0.
The worst scenario is in CNS-acting drugs. In my experience, generic Ativan works less than 10% of the time, so if a patient does not respond to generic Ativan, I am not sure of the cause. I also have found that generic Prozac works less than 50% of the time, and there are treatment problems with generic Wellbutrin. If a patient responds well to the anti-depressant Lexapro, the HMO generally asks me to switch the patient to a generic form of Celexa, even though they are chemically totally different drugs. I have also found that the beta-blocker Nadolol is excellent prophylaxis against migraines, as is Inderal LA, but since only Inderal (which requires dosing 4 x a day) is available in a generic form, the LA Rx. is generally not paid for.
In your own case, I am certain that aspirin, Advil, and Alleve all work differently in you, even though they all are NSAID's (anti-inflammatories).
Viagra, Levitra, and Cialis all work to treat erectile dysfunction, but sometimes one drug will work, and sometimes another.
The main point is that drugs with the same physiological endpoint in the human body have different chemical structures, and they cannot all work equally well.