NMR and UV-vis spectroscopy have been used to study the complexation of antiarrhythmic alkaloid lappaconitine with an efficient complexing agent from licorice, glycyrrhizic acid, which is known to profoundly influence the therapeutic activity of the alkaloid in the complex. In MeOH, DMSO, or aqueous solutions, lappaconitine has been shown to form a stable complex with glycyrrhizic acid with 1: 1 stoichiometry over a broad concentration range from 1 mu M to 300 mu M. The stability constant K-11 equals 2.0 x 10(5) m(-1) in aqueous solution. A similar complex of lappaconitine hydrobromide-the pharmaceutical formulation used in the treatment of arrhythmia-is 2 orders of magnitude less stable than pure lappaconitine. A notable decrease in the rate of the photoinduced electron transfer reaction between lappaconitine in a complex with glycyrrhizic acid and tyrosine allows the suggestion of an explicit interrelation between the suppressed chemical reactivity of the bound alkaloid and the changes of its therapeutic efficiency.