Does bioavailability mean you just need X times more of one method than the other?

Factual Questions
1

Say you have Generic Drug ABC. When administered via IV drip, its bioavailability is 100%. When taken by mouth (pill, tablet, etc.), its bioavailability is 20%.

Does this mean you just need to take 5x as much of the medication via mouth to achieve the same physical effect as 1x of the medicine by IV drip, or is it not that simple?

Or, even for illicit drugs. Say someone wants his heroin. Does injecting vs snorting mean that the higher bioavailability of one means you get the same high for much less drug amount than the other?

2

IANA pharmacologist but…

Not exactly. Oral bioavailability varies patient to patient, so an oral/IV conversion factor of “five times” might be an accurate population figure, but inaccurate for a particular patient (I’m sticking to medicinal drug use here). Age, sex and ethnicity of a patient can be significant factors for bioavailability.

Another important consideration is how drug metabolism works. Low bioavailability of orally administered drugs is frequently a result of high “first pass metabolism.”

a phenomenon of drug metabolism whereby the concentration of a drug, specifically when administered orally, is greatly reduced before it reaches the systemic circulation. It is the fraction of drug lost during the process of absorption which is generally related to the liver and gut wall.

Source

So sometimes “low bioavailability” = “lots of metabolite(s) present”. Pharmacologically active parent drugs frequently have pharmacologically active metabolites. The pharmacological effects of these metabolites could be similar to the parent molecule (in which case high first pass metabolism/low bioavailability may be less of an issue) or the effects may be very different, and thus potentially problematic. So simply throwing five times as much of a drug down a patient’s throat may not be an straightforward/acceptable solution.

When it comes to deciding which dosage forms are appropriate for licensing, you have to take into account where and how a drug will be administered. Self-administration of injected drugs is highly unusual (insulin being an obvious exception).

Taking opioid analgesics as an example, for moderate* pain you probably wish to medicate at home, so an oral dosage for would be required, in which case a low bioavailability may be considered acceptable. For severe pain, it might be the case that analgesia is administered in a hospital setting, intravenously, so that bioavailability is not an issue. A different solution for moderate pain, which sidesteps the high first pass metabolism of most opioids, it to use a different drug. Tramadol is an opioid analgesic with a relatively low first pass metabolism, and oral forms of it are widely used.

Bioavailability can also be complicated by distribution of the drug within the body tissues. But that’s above my pay grade.

j

* - “moderate” is a technical term here. Moderate pain can still be enough to make your life hell.