The development of combinatorial chemistry and high-throughput screening techniques has made it possible to generate many new drug candidates very rapidly, but it has also resulted in a number of poorly soluble and/or poorly absorbable candidates. A new trend in drug development based on pharmacogenomics or the development of molecular-targeted drugs is also spurring the tendency, and it does not necessarily lead to good output in terms of the development of new drugs. It is attractive to improve membrane permeability as well as solubility by using adjuvants, because this method could be applicable for various drugs. However, the practical use of absorption-enhancing adjuvants has been limited because of the potential local toxicity. Therefore suppressing the potential local toxicity would lead to the successful development of safe preparations with improved absorption using adjuvants. Our biochemical and histopathologic studies showed that several amino acids such as taurine and L-glutamine had cytoprotective activity, and it has been found that the combinatorial use of sodium laurate (C12) with these amino acids could maintain the absorption-enhancing ability of C12. A suppository preparation containing C12 and taurine remarkably improved the rectal absorption of rebamipide, classified as BCS class IV, and the preparation was safe to the rectal mucosa. For the mechanisms of cytoprotective action by these amino acids, it has been found that they suppress the intracellular calcium level, induce the expression of heat-shock protein 70, and inhibit the release of histamine and apoptosis.