The term Prabhava (Samskrit: प्रभावः) literally means impact, influence, effect or even some supernatural power of dravya (द्रव्यः | substance). This term in used in Ayurveda in the study of herbs, their activity and effect on bodily functions. Prabhava is one of the characteristics of a dravya (substance) mentioned in Ayurveda along with rasa (रसः), veerya (वीर्यम्) etc. where the term prabhava indicates, action of a certain drug which is irrespective of the other qualities exhibited by a dravya, precisely a herb. A certain factor in a dravya which has overall different pharmacodynamic effect on body is prabhava. Prabhava can also be termed as a special effect of a certain dravya (drug or herb)
प्रभाव परिभाषा॥ Definition
The prabhava of a substance is defined in Charaka Samhita (चरक संहिता) as follows,
रसवीर्यविपाकानां सामान्यं यत्र लक्ष्यते| विशेषः कर्मणां चैव प्रभावस्तस्य स स्मृतः|| (Char. Samh. 26.67)
rasavīryavipākānāṁ sāmānyaṁ yatra lakṣyate| viśeṣaḥ karmaṇāṁ caiva prabhāvastasya sa smr̥taḥ|| (Char. Samh. 26.67)
Meaning: When rasa, veerya, vipaka have properties which are relative to the drug but there is a specific quality that the dravya (drug) exhibits which have a certain effect on our body which is irrespective of rasa, veerya etc, that quality and its action in our body is termed as Prabhava.
According to Ayurveda a drug or dravya is composed of Rasa, veerya, vipaka etc and thus it shows its action accordingly in our body. But when the action of the dravya cannot be explained by its natural composition it is known as prabhava. Thus Prabhava of a substance is also popularly known as its special effect.
प्रभावस्य अनुमानम् तथा निर्धारणम्॥ Determination of Prabhava
Prabhava being a quality present in a dravya (drug) it's determination can only be done by understanding the effects of that dravya in our body. For most of the herbs in Ayurvedic pharmacopoeia prabhava of herb has been already mentioned by ancient Ayurveda scholars. Since prabhava of a substance is not related to its other characteristics, it is difficult to assess and decide prabhava of any herb. Currently the prabhava is understood only with the help of knowledge about that herb given in Nighantu (निघंटु | Materia medica) or samhitas written by ancient Ayurveda scholars. The method by which knowledge about 'Prabhava' of a specific dravya is obtained is known as Aptopadesha pramana (आप्तोपदेश प्रमाणम्)' Prabhava is thus believed to be as incomprehensible.
... प्रभावकृतं तेषां प्रभावोऽचिन्त्य उच्यते ॥ (Char. Samh. 26.70-73) 
... prabhāvakr̥taṁ teṣāṁ prabhāvo'cintya ucyate ॥ (Char. Samh. 26.70-73)
There are various examples mentioned in Ayurveda samhitas (संहिता) to determine prabhava. One such example is, one poison acting as antidote against other poison is due to prabhava effect of that dravya. The action of certain drugs is due to rasa of that dravya, some act according to the veerya, and some according to prabhava. Among all the characteristics, which will be the one to display the final effect of a dravya is decided on the basis of the strength of that characteristic. When all these characteristics are of the equal strength, the dravya acts on the basis of its prabhava.
किञ्चिद्रसेन कुरुते कर्म वीर्येण चापरम्| द्रव्यं गुणेन पाकेन प्रभावेण च किञ्चन॥
रसं विपाकस्तौ वीर्यं प्रभावस्तानपोहति| बलसाम्ये रसादीनामिति नैसर्गिकं बलम्॥ (Char. Samh. 26.70-73)
kiñcidrasena kurute karma vīryeṇa cāparam| dravyaṁ guṇena pākena prabhāveṇa ca kiñcana॥
rasaṁ vipākastau vīryaṁ prabhāvastānapohati| balasāmye rasādīnāmiti naisargikaṁ balam॥ (Char. Samh. 26.70-73)
Meaning: Thus, vipaka, veerya and prabhava are explained well. Some drug acts by (means of) rasa, other by veerya and other by guna, vipaka or prabhava. In case of equality of strength, vipaka subdues rasa, veerya subdues both, and prabhava all these three. These are the natural relative degree of strength
चिकित्सायाम् प्रभावस्य उपयोगिता॥ Application of the concept of prabhava in Ayurvedic medicine
The concept of Prabhava is highly important in Dravya-guna-shastra (द्रव्यगुणशास्त्रम्) of Ayurveda. The activity of various herbs and food articles can be determined on the basis of their rasa, veeryam, vipaka, karma and prabhava. All of these are important to decide the effect of that dravya (substance) on its use as medicine or food. Some herbs as well as food article predominantly act through their Prabhava. Therefore prabhava becomes the crucial factor while selecting herbs and food articles for a vaidya (वैद्यः). Below given are few examples wherein Prabhava is considered to be the chief criteria for selection of few herbs and food articles to get desired activity.
- Chitraka (चित्रक | Plumbago Zeylanica) and Danti (दन्ती | Baliospermum Montanum) are two herbs which have same Rasa, veerya and Vipaka but Danti acts as rechana (रेचनम् | Purgative) where as Chitraka does not. This special effect of Danti is called as Prabhava.
- Tila (तिलः | Sesamum Indicum) and Madanaphala (मदनफलम् | Randia Dumetorum) are again two such drugs or herbs which have same Rasa, veerya, vipaka composition but Madanaphala acts as vamaka (वामकम् | emetic) dravya where as tila does not. 
- Dugdha (दुग्धम् | Milk) and Ghruta (घृतम् | Ghee) are also one of the examples where in the rasa of both these dravya is madhura (मधुरम् | sweet) and Veerya is sheeta (शीतम् | cold), but because of Prabhava Ghruta is Deepan (दीपनम् | kindles digestive fire) and Dugdha is not.
- Madhuyashti (मधुयष्टि | Glycyrrhiza Glabra) and Mrudvika (मृद्विका | Vitis Vinifera) also are two herbs which have same dravya composition but because of Prabhava (special effect) Mrudvika is rechana (रेचनम् | purgative) but Madhuyashti is not.
- There are similarly various Drugs of which their action is justified on basis of Prabhava, [such as Shankhapushpi (शंखपुष्पी | Convolvulous Pluricaulis) acting as Medhya (मेध्यम् | promoting intellect), Shirisha (शिरीषम् | Albizia Lebbeck) as Vishaghna (विषघ्नम् | neutralizing poison or antidotal activity) etc.]
प्रभावस्य सार्थकता॥ Importance Of Prabhava in Ayurvedic Pharmacology
Prabhava as a concept is important in Ayurveda because when it comes to dravya (drug), the rasa etc qualities are the composition of dravya (drug) but prabhava is the inbuilt nature of some dravyas (drug). Thus, from the standpoint of ayurvedic pharmacology, prabhava is one such important aspect.
There also is mention of precious stones which when worn according to their properties have shown effects of body, mind, health and well-being; which are stated due to the action of Prabhava. The entire set of knowledge about wearing these precious stones and crystals for some desired effect is known as Gemology in contemporary times. However in Ayurveda and other bharateeya shastras (भारतीय शास्त्राणि) use of these precious stones in ornaments for specific benefit has been mentioned thousands of years ago. This activity of certain metals and stones is justified by the concept of prabhava explained in Ayurveda.
According to some acharyas when a dravya (drug) acts based on rasa, veerya (potency) or vipaka it counters the dosha imbalance in our body and affects dhatus separately. However prabhava is one such quality of a substance precisely a herb or a medicinal substance, which is believed to act directly on the Dosha-Dhatu complex (दोष-दूष्य संमूर्छना) of a certain disease at a time. Thus when a dravya (drug) acts based on prabhava it directly counters the disease per se. Therefore there are many such herbs and food articles that have prabhava specific to some diseases (directly acting to cure a certain disease) which makes them drug of choice to treat those illnesses.
- Charaka Samhita (Sutrasthanam Adhyaya 26 Sutra 67)
- Charaka Samhita (Sutrasthanam Adhyaya 26 Sutras 70-73)
- Available from Charakasamhitaonline.com
- Charaka Samhita (Sutrasthanam Adhyaya 26 Sutra 68)
- Ashtanga Samgraha (Sutrasthanam Adhyaya 17 Sutras 53-59)
- Ashtanga Samgraha (Sutrasthanam Adhyaya 17 Sutra 51)