Nonmedical use of sildenafil citrate (SC) requires new methods for drug determination in human serum and in tablets. SC as standard and in Sildena (R) tablets is determined by square wave voltammetry (SWV) in 0.1 M H2SO4 (harmful for human serum) at gold electrode in a range: (1 x 10(-3),1 x 10(-2), 0.1, 0.5,1) mu M and on cysteine (Cys) modified gold electrode (Au/Cys) in a range: (1 x 10(-3),1 x 10(-2), 0.05, 0.1) mu M. The presence of Cys causes two times larger peak currents and shifting of the incipient potential of the SC oxidation to 0.1 V in negative direction. A safe procedure for SC determination as standard and in Sildena (R) tablets spiked with human serum is developed by SWV at gold electrode in 0.05 M NaHCO3 (suitable for human liquids) in a range: (1 x 10(-3),1 x 10(-2), 0.1, 0.2, 03) mu M. Microscopic characterization of the surfaces morphology was also performed. The SC concentrations were checked by high performance liquid chromatography-ultraviolet spectroscopy (HPLC-UV) showing that electrochemical method is more sensitive and can be used for the measurements of very low concentrations of the analyte. (C) 2016 Elsevier B.V. All rights reserved.