My brother takes Effexor (venlafaxine) but thinks he has some stomach absorption problems. Could we test this hypothesis, say through a blood test?

No Effexor (venlafaxine) test. There is no test to see if specifically Effexor (venlafaxine) is not being absorbed properly but if he truly has absorption problems- as in calorie, fat, vitamins, protein, carbohydrates-there would be various other symptoms and signs-too numerous to go into here, but these do have various tests. There does not exist a blood test to test for drug malabsorption.

Related Questions

My brother takes Effexor (venlafaxine) but thinks he has some stomach absorption problems. Could we test this hypothesis, say using blood tests?

Better than that. See your doctor or a GI specialist and they will tell you straight away if there are possibilities of stomach absorption problems or not. I'm curious to know how your brother came to the conclusion that there must be problems with gastric absorption? Read more...