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Virya, usually translated as potency, is the property by which Ayurveda explains a drug’s main action. In the dominant two-fold scheme, drugs are either ushna (hot) or sheeta (cold). Hot drugs are said to promote digestion, sweating and dissolution of accumulations, and cold drugs to soothe, nourish and stop bleeding. The concept parallels the hot and cold properties of traditional Chinese medicine and the temperaments of Persian medicine. We review how virya can be tested. At the molecular level, the discovery of thermosensitive transient receptor potential (TRP) channels showed that the heat receptor TRPV1 responds to capsaicin and the cold receptor TRPM8 to menthol, providing a mechanism by which plant compounds evoke sensations of heat and cold. At the physiological level, a meta-analysis of human studies found that capsaicin and capsiate modestly increase energy expenditure and fat oxidation. A three-month randomised trial of ginger extract, by contrast, found no change in resting energy expenditure. Research on Chinese hot herbs, such as aconite, dried ginger and cinnamon, has linked them to activation of brown adipose tissue and thermogenic gene expression, and cold herbs to antipyretic effects through TRPM8 and fever mediators. Research on Ayurvedic virya itself is sparse and has used indirect measures such as the thermal diffusivity of oils. We propose a set of physiological endpoints for testing ushna and sheeta virya, from sensory and TRP-channel assays to energy expenditure, thermoregulation and digestive function, and recommend comparative studies of drugs of opposite virya under standardised conditions.