Quote:
Originally Posted by Shaani 
so am i right in thinking that because the drug e.g. Salbutamol goes through the Gastro-intestinal system, there's more chance that some of it will not be absorbed (and therefore excreted) hence why a larger dose is required than if inhaling Salbutamol cos its going more or less directly into the blood stream via the lungs (no digestion and more drug absorbtion taking place)?
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Hi,
The majority of the ingested salbutamol is absorbed. Salbutamol is mainly hepatically metabolised (basically; the salbutamol is broken down in the liver). The anatomy of the gastro-intestinal tract means the the blood flow from the stomach and intestines goes to the liver, then on into the inferior vena cava, into the heart, round the lungs and then back to the heart, then off to the rest of the body. This is important because lots of the absorbed salbutamol is metabolised when it first passes through the liver. Giving the salbutamol via an inhaled process means it misses this first pass metabolism and hence the dosage required is smaller (although not entirely true because salbutamol has a topical effect in the lungs).
Some medicines have major first pass metabolism. Propanolol is nearly 90% first pass metabolised. Given orally, we use doses of 80-360mg/day but when given intravenously (where 100% gets into the blood) we use doses of 5-10mg.
Hope this ain't too confusing?!
