Carbon dioxide inhalation (rofecoxib and celecoxib): This study is identical to the one above, except either rofecoxib or celecoxib is given instead of indomethacin. Both of these drugs are also non-steroidal anti-inflammatory drugs that inhibit prostaglandin production. Unlike indomethacin, rofecoxib and celecoxib are given orally instead of through a vein, so, to allow time for the drug to be absorbed, the second scan is delayed for 2 hours.