MT-CO1 and duodenal ulcer: NSAIDs classification according to selectivity to COX-1 and COX-2 is as follows: (1) COX-1 selective inhibitors (low-dosage aspirin); (2) COX non-selective inhibitors (the majority of classified NSAIDs, which when administered over the long term, e.g., in cases of rheumatoid arthritis, cause duodenal ulcers in 20% of cases and gastric hemorrhage in 1–4% of cases/year); (3) COX-2 preferential inhibitors (meloxicam and nimesulide, which have fewer gastric side effects than standard NSAIDs, but which are not risk-free at high doses); (4) COX-2 selective inhibitors (celecoxib and rofecoxib) [20].