This paper reports the influence of organic additives (alcohols, amino acids, sugars) on the micellization and cloud point (CP) of a phenothiazine drug, promethazine hydrochloride (PMT). The critical micelle concentration (CMC) of the drug, determined by surface tension measurements in the presence of a representative of each additive class (i.e., butanol, leucine, arabinose), are used to evaluate the maximum surface excess concentration (Γ max) and the minimum area per surfactant molecule (A min) at the air/water interface. Γ max increases and CMC/A min decreases with increasing concentration of the additives, which indicate mixed micelle formation. The intermicellar interaction coefficients in the mixed micelles (β m and β σ are also calculated, and their negative values imply attractive interactions. Effect of pH revealed CP decrease with increasing pH due to deprotonation of PMT molecules. Effect of amino acids depended upon their nature and polarity, whereas sugars caused a CP decreasing effect. Aliphatic alcohols as well as cycloalkanols and diols decreased the CP. In the presence of arabinose, increase in drug concentration resulted in the CP increase, while increase in pH showed an opposite trend. Results are interpreted on the basis of mixed micelle formation, hydrophobic interactions, and change in solvent structure.