Anything is possible. Some people have a genetic need for more thiamin than others.

I would try supplementing with up to 300mg of thiamin a day in divided doses when using Lasix.

There is a better form, that stays in the body longer which we on PN forum use. It is called benfotiamine.

Here is an example of a good brand to try:
http://www.iherb.com/Doctor-s-Best-B...Veggie-Caps/42

Most of the PNers start at 300mg a day and reduce to 150mg over time as a maintenance dose.

Lasix is very hard on the body, as far as nutrient depletion goes.
Thiamine is not the only thing it affects, as that link I gave above shows.

I'd make sure he has adequate and often kidney tests as well. Also thyroid should be checked as edema is one sign of hypothyroidism.

Gabapentin can cause carbohydrate cravings. Carbs tend to hold water in the body as well. People vary in this response. A person consuming many calories in carbohydrate/sugar content will use up thiamin quite a bit, as thiamin is a cofactor in carb metabolism. Also if he drinks alcohol, the thiamin gets used up quickly that way.

More on thiamin:
http://lpi.oregonstate.edu/infocenter/vitamins/thiamin/