Screening suspected counterfeit Viagra® and imitations of Viagra® with near-infrared spectroscopy

We describe a near-infrared spectroscopy (NIRS) method for fast-screening Viagra® tablets, counterfeit Viagra tablets, and imitations of Viagra. The method can (1) check the homogeneity of a batch; (2) distinguish counterfeits and imitations from authentic Viagra®; (3) screen for the presence of sildenafil citrate, the pharmacologically active substance in Viagra®, irrespectively of the excipients present; (4) and detect whether similar samples have been previously analysed. We applied the method to 103 samples with a diversity of appearance, chemical composition, and origin. Other analytical methods confirmed the positive screening results for sildenafil citrate and the presence of other pharmacological active substances. The NIRS screening indicated the absence of sildenafil citrate in the presence of another pharmacological substance for only 2 samples, where the reference methods showed the presence of sildenafil citrate in addition to that of clomifene citrate. Otherwise, the method gave no false positive or negative results. The NIRS screening method is very fast and reliable for detecting counterfeits and imitations, and it correctly predicts the presence or absence of sildenafil citrate in 98% of the samples.