As the usage of antibiotics and vitamin tablets becomes more widespread, detrimental impacts on living beings are increasing by consuming polluted water contaminated with drug residues. Because of the emergence of drug-resistant bacteria, there is an increasing requirement for determining frequently accessible medicines in water sources. We have developed a rapid colorimetric sensor for the selective recognition of neomycin and thiamine. A simple reduction method was followed to synthesize citrate capped silver nanoparticles (AgNPs) as a sensor. The presence of functional groups and the surface charge of AgNPs were characterized by FT-IR and zeta potential measurements. The colloidal solution of AgNPs displayed an instantaneous chromatic change from yellow to purple and brown-red after the respective treatment with neomycin and thiamine, although the other drugs showed infectivity. The plasmonic characteristic peak at a 405 nm wavelength of AgNPs underwent a gradual decrease with an increase in the concentration of the drug. In addition, it showed a bathochromic shift and broadening to longer wavelengths of 520 nm and 565 nm after the incubation with neomycin and thiamine, respectively. The simultaneous quenching and shift of the plasmonic peak were ascribed to the aggregation of AgNPs in the presence of the analytes via hydrogen bonding and proved by spectroscopic and microscopic techniques. The colloidal AgNPs displayed an excellent performance for the determination of these drugs at concentrations ranging between 7.5 nM and 0.05 mM with low detection limits of 70.8 pM and 0.42 nM for neomycin and thiamine, respectively. The selectivity towards neomycin and thiamine was prolonged even in the presence of interfering drug mixtures and in real sample analysis.