The difference between nasal & rectal doses is that with a nasal dose you're putting a dry powder straight onto a mucous membrane, which because of the limited moisture available, results in a concentrated solution in contact with the membrane, both in terms of acitity of the solution and in terms of the vasoconstrictive effects (they're the two processes that damage the membrane and make is really inflammed). Rectally, it's administered already in solution, and is much lress concentrated when in contact with the rectal mucosa, resulting in minimal, if any, damage to said membrane. Adittionally of course, there's no vile tasting drip when it goes up your arse (at least god got that right by not putting taste buds in the colon! Imagine if he had )