This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
In some conditions of the nervous system, absorption takes place much more slowly than others; indeed, both digestion and absorption appear to be sometimes totally arrested. Thus in persons in whom a sick headache comes on some time after a meal the contents of the stomach are vomited after a while and the food is found to have undergone digestion but not absorption. If the meal be taken after the headache has come on it will be found, in some persons at least, that the food is vomited almost unchanged, both digestion and absorption appearing to be arrested. This condition exists also in delirium tremens, and in a case of this disease I have seen pieces of food thrown up in an undigested condition although they have been swallowed, as the patient has informed me, three or four days before. It is probable that in these conditions drugs are also not absorbed, and I think it is not improbable that the harmlessness of large doses of digitalis given in cases of delirium tremens is due to the non-absorption of the drug.