The story of drugs goes back to the Prehistoric days. The Indian sages invented drugs and their therapeutic uses long ago. This was based partly on observation of the effect of drugs on various animals and partly on human trial and error. Rgveda, the oldest document of Indian wisdom, contains material which shows the rational attitude towards plant kingdom and its exploitation for the benefit of the humanity. The Atharvaveda has got more advanced picture and a larger number of drugs used in a similar number of diseases. On this long tradition and accumulated wisdom the ancient sages, Atreya and his disciples, after a deep and concentrated effort were able to make some generalisations for rational explanation of drug action which formed the basic concepts of dravyaguna. Evidently this study was going on in different circles of scholars and as such different schools of thought arose. The school of Susruta has some differences with Atreya's teachings which was followed by Nagarjuna.
The theory of Panca Mahabhuta is the ultimate basis on which the fundamentals of Dravyaguna stand. This is again based on the law of Uniformity in constitution of material bodies including human body on one side and drugs on the other. How to know the Pancabhautic compositions of a drug particularly in its pharmcodynamic aspect? This was attempted in two ways : one by linking the Rasa as indicator of the Bhautic composition and secondly by observing the sum total effect on the body. Rasas were perceived directly and their effects on the Dosas were also stand-ardised. Then by law of similarity and dissimilarity their relation with Bhutas were known by Inference. Thus Rasas are not direct indicators of drug composition but indirect ones through the channel of Dosas. For instance, Madhura Rasa increases Kapha which is Parthivapya in nature, therefore by law of similarity (If there is increase, there is similarity) Madhura Rasa is taken as Parthivapya and the drug is also taken as Parthivapya. This is again confirmed by law of dissimilarity as it decreases Pitta and Vata which are dissimilar to Ap and Prthvi respectively.