Here, we show a new, simple, and rapid SYBR Green-based Real-Time PCR assay for the quantification of hallucinogenic plants in plant mixtures. As a test plant, Salvia divinorum Epling & Játiva-M., a perennial herb belonging to the Lamiaceae family able to induce hallucinations, changes in perception, or other psychologically induced changes with similar potency as LSD, was used. The method was tested on seven mixtures 100/0%, 80/20%, 60/40%, 40/60%, 20/80%, 10/90%, 0/100% (w/w) S. divinorum versus a non-hallucinogenic plant, Salvia officinalis. Total DNA was extracted from samples and quantified by Real-Time PCR. Arabidopsis thaliana genomic DNA was added, as internal standard, at the beginning of each extraction. A new formula for the interpretation of Real-Time PCR data, based on the relative quantification of DNA extracted from mixture versus a reference DNA extracted from a known amount of pure S. divinorum, was developed. The results of this work show an almost perfect correspondence between Real-Time PCR-calculated weight and the weight estimated by an analytical weighted method, proving the effectiveness of this method for the quantitative analysis of a given species in a plant mixture.