This weakens the argument. If the amount of this chemical in grapefruit juice varies widely from glass to glass, then the effect of a given glass of grapefruit on the effectiveness of a dose of medication will also vary widely. Since this variation increases the chances of getting the wrong dose, which is dangerous, this undermines the claim that prescribing lower doses of medication together with set doses of grapefruit juice is the \"best\" medical approach.

Irrelevant. The author's conclusion about the \"best medical approach\" isn't based on the cost of either grapefruit juice or of medication, just on the effects grapefruit juice has on the absorption of certain medications.

This doesn't weaken the argument. It just reiterates something the premises have already told us: that this chemical in grapefruit juice does affect the absorption of these medicines. If you thought this could weaken by suggesting that it might be a better medical strategy to prescribe the chemical itself, instead of grapefruit juice, with these medications, this would rest on several assumptions: that the chemical has the same effect on its own, without the other compounds in grapefruit juice; that there is a consistent and effective way to remove the chemical and administer it medically; and so on.

We already know this chemical has an effect on absorption of medications. Knowing how the chemical achives that effect doesn't weaken the argument about whether or not we should take advantage of this effect to prescribe lower doses of medication.

This just tells us that doctors in the past might have been aware of grapefruit juice's ability to amplify the effects of medication. This doesn't weaken the author’s recommendation of prescribing lower doses of medication alongside grapefruit juice.