Prabhava in Ayurveda is the potency of a drug. It is a conceptual part of Ayurvedic pharmacology and is discussed elaborately in Ayurvedic classes. Generally, a drug is supposed to act in a particular manner owing to its possession of what are called rasa (essence), guna (property), virya (energetic principle) and vipaka (altered state of drug). Actions are attributed corresponding to each one of these factors. If a given drug acts in a more efficient manner without any regard to the factors cited above and does not correspond to, and cannot be attributed to, the presence of either f the four named above, then a concept like prabhava become necessary to explain the cause of action.
Agnivesha, in his medical compendium, defines prabhava as the distinctive feature responsible for some specific effects not supported by taste, vipaka ad potency possessed by two drugs similar to each other. Superseding the nature of a drug, prabhava acts in a specific manner. For example, if two drugs are similar in taste and other aspects, and if one is purgative, while the other is not, the purgative action of one drug is considered as prabhava. Likewise, the antitoxic effects of antitoxins, aphrodisiac drugs, purgatives and emetics, the actions of which are beyond certain reasoning , are all explained by the term prabhava.
Thus it has been found that the potency of the roots and other parts of medicinal plants depends upon the locale and season of their procurement. The roots collected from dry ground have more potency than those collected from marshy places. To be potent, leaves have to be collected during the rainy and spring seasons, roots during summer or winter, and barks and tubers in autumn (Charaka Samhita, Kalpa-sthana 10-12)