What is the one word for loss in trade / bad deal?

What is the one word for a deal that will end in loss for sure

for example a teenager doing drugs over studies is a sure loss to him how do I say that ? He will regret is one way , but my point is to emphasize that he is investing in the wrong thing.

Solution 1:

As in: TFD blind alley - a noun

I tell X that the path he/she has taken is a blind alley.

2. A mistaken, unproductive undertaking.

Solution 2:

A teenager doing drugs over studies is backing a losing horse.

From McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. :

back the wrong horse.
Fig. to support someone or something that cannot win or succeed.

I don't want to back the wrong horse, but it seems to me that Jed is the better candidate.
Fred backed the wrong horse in the budget hearings.

You can also say betting/backing on the wrong (or losing) horse.