Aspirin remains in unionised form in acidic $pH$ but this unionised form is a diffusible form which is helpful in absorption across lipid membrane.
My reasoning almost tries to contradict the statement and find a suitable answer to the question as to Why aspirin doses are reduced in gastric achlorydia ?
Here are some issues I need to resolve.
In gastric achlorydia there must be more ionic form of aspirin inside stomach so shouldn't we increase the dose to get more diffusible form i.e unionised form of aspirin for absorption to reach a specific plasma concentrations level considering that absorption also occurs from stomach?
Somewhere it was also mentioned (in my textbook) that solubility of aspirin increases at higher $pH$ but I am not sure whether this is the reason for why the dose of aspirin should be reduced in achlorydia? cause some people suggested me that it was because of no ion trapping and hence more of the aspirin is available for absorption from intestine so dose must be reduced. In fact this question has got in my nerves and I don't know which is the correct answer/more significant one. The $pH$ of intestine is basic which might give more ionisable form so decrease in absorption according to me but still I am not pretty confident to rule out the reason (related to ion trapping).
I am adding this to tell how I came up with this question and ended up using this website.
After I googled there was no such direct question mentioned above why aspirin dose are reduced in hgastric achlorydia?. This question was asked to us in practical classes and the answer given to us was that '...since in gastric achlorydia there is more ionisation of aspirin and hence no ion trapping so more of drug is available for absorption through intestine hence we reduce the dose of aspirin to get a specific plasma concentration level.' Ionic form of aspirin is more better absorbed in intestine than in stomach relatively as given here.
But on the other hand our book states
Aspirin is absorbed from the stomach and small intestine. It's poor water solubility is limiting factor in absorption: microfilming there drug-particles and inclusion of an alkali(solubility is more at higher pH) enhances absorption. However, higher pH also favors ionisation, this decreasing the diffusible form.
- Essentials of Medical pharmacology 7th edition, NSAIDS and antipyretic analgesics, Pharmacokinetics, page no. 196.
Now here it mentions that the solubility is limiting factor so possibly it may increase available small aspirin particles for absorption at high pH in gastric achlorydia therefore enough to counter the effect of increased ionic form which decrease absorption.
I don't know which is correct explanation or if both correct, which one is more significant.