Bioavailability

The FDA defines bioavailability as:

"the rate and extent to which the active ingredient or active moiety is absorbed from a drug product and becomes available at the site of action."

Basically, anything that administers directly into the bloodstream through an IV has instantaneous bioavailability for your system, whereas pills taken orally must absorb through the body's natural digestive processes. Some medications are more successful at this than others.

The FDA defines bioequivalent as:

"the absence of a significant difference in the rate and extent to which the active ingredient or active moiety in pharmaceutical equivalents or pharmaceutical alternatives becomes available at the site of drug action when administered at the same molar dose under similar conditions in an appropriately designed study."

Generic versions of existing drugs need to prove they have similar bioavailability to established brands. The American Heart Association website has an article elaborating on this process; the generic must show:

"absorption parameters generally falling between 80% and 125% of those obtained with the proprietary agent under the same testing conditions. The use of the –20%/+25% rule is based on a regulatory decision that for most drugs that difference in concentration of the active ingredient in blood will not be clinically significant."

As the AHA article discusses, sometimes a small degree of variation in the drug can produce significant effects in the patients' health. This may arise from trace elements present in one drug and absent in another. This variation comes during the transition from the innovator drug (with clinical trials and many studies to define the bioavailability and the therapeutic efficacy) to the generic version produced years later. The FDA stance is if the primary drug ingredient meets the standards for the bioequivalence measure, i.e. 10 grams in one medication is bioequivalent to 8-12.5 grams in the generic version, the medical efficacy should be the same and tighter measures would make it hard for manufacturers to produce drugs bioequivalent to themselves.

The AHA article states this is obviously critical for life-threatening treatments and not so much for other therapies, but nevertheless makes a bold statement:

"Our conclusion is that although generic drug approval requires demonstration of chemical equivalence and bioequivalence as defined above, published reports and our survey indicate that these equivalencies do not guarantee therapeutic equivalence, which is what caregivers desire and patients expect."

I presumed generic drugs were exactly the same as the branded counterparts. I'm not alone, I know, as the AHA article points out:

"Only 17 percent of physicians could correctly identify the FDA standards for bioequivalency."