Bg 18.37

yat tad agre visam iva
pariname ’mrtopamam
tat sukham sattvikam proktam
atma-buddhi-prasada-jam

Word for word: 
yat — which; tat — that; agre — in the beginning; visam iva — like poison; pariname — at the end; amrta — nectar; upamam — compared to; tat — that; sukham — happiness; sattvikam — in the mode of goodness; proktam — is said; atma — in the self; buddhi — of intelligence; prasada-jam — born of the satisfaction.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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 by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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.