literally it may not be true but there comes a point where not enough of a drug is absorbed to have any noticable effect and thats the cut off
bodies dont have absolutes
Sure the bioavailability may approach zero as the dose increases, but what I meant is that, given a linear relationship with negative slope between bioavailability and dose, there is a point where zero of the drug (zero in absolute quantity, not bioavailability) get absorbed.
I don't think anyone would argue that there is some large dose of gabapentin that has
ess of an overall effect (again, talking about
absolute quantity of drug absorbed) than a smaller dose. This is different than saying that the
increase in dose has no noticeable effect, which is likely true.
With a high enough dose, the amino acid transporters will be saturated and the gabapentin should theoretically absorb at a constant rate independent of dosage. How could it be possible, then, that further increasing the dose beyond this would
decrease the
absorption rate (absorption rate, NOT bioavailability)?
Say you double the dose beyond the minimum dose needed to saturate the transporters. The transporters will still be saturated, then, the absorption rate should be roughly the same, and the excess gabapentin from the larger dose will be excreted. The
increase in dose may have no noticeable effect, but the larger dose will still have roughly the same effect as the smaller dose. This describes an inversely proportional bioavailability-dose relationship.
The only possible mechanism I can think of that would decrease absolute quantity of absorbed gabapentin with a sufficiently high dose is that somehow, too much gabapentin would literally destroy your amino acid transporters, in which case you would probably die anyways. I doubt this happens within a few orders of magnitude of a typical medical or recreational dose, considering gabapentin's safety record, however. If this effect happened at typical doses and was responsible for the negative correlation between bioavailability and dose, then you would expect gabapentin to be very toxic in medical doses.
On another note, saying `I intend linear to be synonymous to inversely proportional' makes about as much sense as "I intend orange to be synonymous with apple".