Therefore, erythromycin has to be given in a correspondingly larger dose. The keto-enol tautomers of curcumin-based molecules were potent at both BACE-1 and GSK-3β targets, while diketo tautomer-based molecules were less active or inactive. In some cases, tautomers exist in equilibrium with cationic, anionic, and zwitterionic forms, and it is difficult to determine which form may be responsible for drug action. For example, edaravone is present in 50% anionic form and the rest as a mixture of three neutral tautomeric forms (Figure ). Avobenzone is the only FDA-approved ultraviolet filter molecule that absorbs harmful UVA radiation. The protective effect and stability of this molecule is tautomer-dependent. The keto-enol tautomer protects from UVA, while diketo tautomer is susceptible to photodegradation. 11