The mode of action of the non-steroid anti-inflammatory drugs (NSAIDs) is attributed primarily to the inhibition of PG synthesis, and more specifically, to the inhibition of the COX enzyme system. In an effort to gain a deeper insight on the properties of NSAIDs as Indometacine, {l-(4-chlorobenzoyl)-5-methoxy-2-methyl-1H-indole-3-acetic acid}; Diclofenac, {[2-(2,6-dichlorophenyl) amino]-benzeneacetic acid}; and Niflumic acid, {2-3((3-trifluoromethyl)phenylamino)-3-pyridinecarboxylic acid}, that will provide knowledge of their action, we have performed theoretical studies in the frame of the AIM Bader's theory (atoms in molecules). The topological properties of the NSAIDs studied show that the C–C, C–N and C–H ring bonds are typical of covalent interactions. The C
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O and O–H bonds in the carboxylate groups are strong shared interactions. Hydrogen bonds are only localized in the Niflumic acid. The oxygen atoms of the carboxylic groups are sites of the highest concentration of charge and it is assumed that these atoms shall be the preferred sites for an electrophilic attack.