यत्तदग्रे विषमिव परिणामेऽमृतोपमम्। तत्सुखं सात्त्विकं प्रोक्तमात्मबुद्धिप्रसादजम् ॥

yat tad agre viṣam iva pariṇāme 'mṛtopamam tat sukhaṁ sāttvikaṁ proktam ātma-buddhi-prasāda-jam

Which at first is as venom but in the end is as nectar; that pleasure is said to be pure, born of the blissful knowledge of the Self.

In plain words: Pure happiness feels like poison at first but becomes like sweet nectar in the end. It comes from clearly knowing your true Self.