In Āyurveda treatments are of two types: Samśodhanacikitsāa and Saṃśamanacikitsāa. Samśodhanacikitsāa involves the pañcakarma treatments and is considered to be the superior to saṃśamanacikitsāa. Pañcakarma is a specially designed five procedures for internal purification of the body through the nearest possible route. It has a whole role in promotive, preventive and curative aspect. Virecana is one among the Pañcakarma and Pañcaśodhana. It is considered as an ideal treatment for the elimination of Pitta doṣaḥ and also beneficial for diseases which pitta is associated with vāta or kapha. Virecana karma is practiced as a śodhana therapy in routine clinical practice. 

ETYMOLOGY

According to vācaspatya: term virecana has three components

Vi- upasarga

Ricir-Ric dhātu

Lyut- pratyaya

According to Śabdakalpadruma: ‘Rici’-dhatu.’Vi’-upasarga with ‘Ṇic’ and ‘Lyut’- pratyaya.‘lyutviśeṣenarecayati’. The root Rici dhatu, Viupasarga with ‘Nic’ and ‘lyut’ partyaya gives the meaning Viśeṣenarecayatiti.

विरेचनशब्दार्थं विभजते- अधो गुदेन दोषनिर्हरणं भजत इति अधोभागम्| वमनविरेचनयोरपि कदाचिद्विरेचनसञ्ज्ञां षड्विरेचनशताश्रितीयोक्तां दर्शयन्नाह- उभयमपीत्यादि|                                       (Cha. Ka. 1/4 chakrapani commentary)

The word virecana is commonly used for evacuation. As both vamana and virecana do the evacuation of doṣas, sometimes virecana broadly applies for both. But generally, virecana indicates the evacuation of doṣas through guda.

DEFINITION

तत्र दोषहरण, अधोभागं विरेचनसञ्ज्ञकम्; 

उभयं वा शरीरमलविरेचनाद्विरेचनसञ्ज्ञां लभते||       (Cha. Ka. 1/4)

Expulsion of doṣas through the adhobhāga is virecana. ĀcāryaCakrapāṇi comments Adhobhāga’ to Gudamārga. In certain circumstances expulsion of doṣas through both Urdhva and Adhobhāga are termed to be known as Virecana. 

पित्तेतुविरेकंश्लेष्मसंसृष्टेवातत्स्थानगतेवाश्लेष्मणीति॥                   (A. S. Su. 27/4)

Virecana is administered for vyādhis vitiated due to pitta, pitta associated with kaphaand also in pitta sthānagatakaphajavyādhi.

विरेचनं पित्तहराणां||                                        (Cha. Su. 25/40)

Virecana is a śreṣṭha pitta hara cikitsā.

विरेचनं तु सर्वोपक्रमेभ्यः पित्ते प्रधानतमं मन्यन्ते भिषजः; तद्ध्यादित एवामाशयमनुप्रविश्य केवलं वैकारिकं पित्तमूलमपकर्षति, तत्रावजिते पित्तेऽपि शरीरान्तर्गताः पित्तविकाराः प्रशान्तिमापद्यन्ते, यथाऽग्नौ व्यपोढे केवलमग्निगृहं शीतीभवति तद्वत्||१६||

(Cha. Su. 20/16)

Virecana is the main treatment for pitta doṣa. It does the pitta doṣanirharana. Āmāśaya is the sthāna for pitta and kaphadoṣa. It has been told by a similie, that, how a building caught fire and the fire is cooled by subsiding it, in the same way the āmāśayagata pitta and all other pittas gets subsided by virecana karma.

बुद्धेः प्रसादं बलमिन्द्रियाणां धातुस्थिरत्वं बलमग्निदीप्तिम् |
चिराच्च पाकं वयसः करोति विरेचनं सम्यगुपास्यमानम् ||
यथौदकानामुदकेऽपनीते चरस्थिराणां भवति प्रणाशः |
पित्ते हृते त्वेवमुपद्रवाणां पित्तात्मकानां भवति प्रणाशः ||        (Su. Chi. 33/27-28)

By virecana karma, it does buddhi prasādana, does indriyabala, dhatu sthiratva, balavardhaka, does agnidīpti, delays ageing process. By the removal of water, how all the reliants of water gets destroyed. In the same way, by the removal of pitta doṣa through virecana, it gets rid of all the diseases caused due to pitta.

Why Vamana is prior to Virecana 

विरेचनमपि स्निग्धस्विन्नाय वान्ताय च देयम्; अवान्तस्य हि सम्यग्विरीक्तस्यापि सतोऽधः स्रस्तः श्लेष्मा ग्रहणीं छादयति, गौरवमापादयति, प्रवाहिकां वा जनयति ||                                                                                                       (Su. Chi. 33/19)

Virecana is administered to a person who has undergone snehana, svedana and vamana. In avānta, if samyakvirecana has occurred, it is said that, it draws the kapha from the āmāśaya downwards and covers the grahaṇī and causes gauravata and pravāhika. So to better avoid such complications, vamana has to be done prior to virecana.