Such happiness (which is in the beginning may be very pleasing to the senses, but at the end, or after some time, which becomes just like poison) is always in the mode of passion. BG 1972 purports

Expressions researched:
"Such happiness is always in the mode of passion"

Bhagavad-gita As it is

BG Chapters 13 - 18

A young man and a young woman meet, and the senses drive the young man to see her, to touch her and to have sexual intercourse. In the beginning this may be very pleasing to the senses, but at the end, or after some time, it becomes just like poison. They are separated or there is divorce, there is lamentation, there is sorrow, etc. Such happiness is always in the mode of passion. Happiness derived from a combination of the senses and the sense objects is always a cause of distress and should be avoided by all means.

BG 18.38 (1972), Translation and Purport

That happiness which is derived from contact of the senses with their objects and which appears like nectar at first but poison at the end is said to be of the nature of passion.

A young man and a young woman meet, and the senses drive the young man to see her, to touch her and to have sexual intercourse. In the beginning this may be very pleasing to the senses, but at the end, or after some time, it becomes just like poison. They are separated or there is divorce, there is lamentation, there is sorrow, etc. Such happiness is always in the mode of passion. Happiness derived from a combination of the senses and the sense objects is always a cause of distress and should be avoided by all means.