Bg 18.37

yat tad agre viṣam iva
pariṇāme ’mṛtopamam
tat sukhaṁ sāttvikaṁ proktam
ātma-buddhi-prasāda-jam
Synonyms: 
yat — which; tat — that; agre — in the beginning; viṣam iva — like poison; pariṇāme — at the end; amṛta — nectar; upamam — compared to; tat — that; sukham — happiness; sāttvikam — in the mode of goodness; proktam — is said; ātma — in the self; buddhi — of intelligence; prasāda-jam — born of the satisfaction.
Translation: 
That which in the beginning may be just like poison but at the end is just like nectar and which awakens one to self-realization is said to be happiness in the mode of goodness.
Purport: 

In the pursuit of self-realization, one has to follow many rules and regulations to control the mind and the senses and to concentrate the mind on the self. All these procedures are very difficult, bitter like poison, but if one is successful in following the regulations and comes to the transcendental position, he begins to drink real nectar, and he enjoys life.