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In Dravyaguna, prabhava is the specific action of a drug that cannot be predicted from its rasa, guna, virya and vipaka. Charaka illustrates it with pairs of drugs that share these properties but differ in action. Danti (Baliospermum montanum) is purgative while Chitraka (Plumbago zeylanica) is not, although both are pungent in taste and vipaka and hot in virya. Draksha is laxative while Madhuka is not, and ghee stimulates digestion while milk does not. Classical texts regard prabhava as achintya, knowable only by observation and authority, not by reasoning from properties. This review examines whether modern pharmacology can explain classical examples of prabhava and what such explanations mean for Dravyaguna. For the Danti–Chitraka pair, Danti root contains phorbol esters, diterpenoids with irritant and purgative potential, whereas the characteristic constituent of Chitraka is the naphthoquinone plumbagin. Different molecular classes can thus produce different actions despite similar sensory properties. More generally, specific actions often arise from minor constituents acting on defined molecular targets, such as piperine on P-glycoprotein and CYP3A4. Sensory properties do not capture these actions, and the hot herbs of Chinese medicine likewise share an outcome but differ in their molecular targets. We argue that prabhava marks the boundary between what a sensory and functional classification can predict and what requires knowledge of specific constituents and targets. Far from being a mystical residue, it is the point at which Dravyaguna explicitly calls for empirical observation. This makes classical prabhava examples useful starting points for discovering specific molecular mechanisms.