That we do not wish

Please post the whole sentence.

thanks GPY

"The book speaks about a few undesirable toxins"
OR
"The book speaks about a few undesired toxins"

Anonymousthanks GPY"The book speaks about a few undesirable toxins"OR"The book speaks about a few undesired toxins"

"undesirable" seems the better choice.

I was going to question whether you really need to say "undesirable" (I mean, as opposed to what? Desirable toxins?) However, on balance I think it's OK.

The principal noun that "undesired" works with is "effect."
Prescribing too many antibiotics has had the undesired effect of creating drug-resistant bacteria. (It has already happened, and it was not appreciated.)

Undesirable is much more common:
The technique of animal training is to reward desirable behaviors and discourage undesirable behaviors.

Thank you GPY and AlpheccaStars for your thorough explanations!
(I hope the above sentence is correct)

I see what you mean, GPY. Toxins are never desirable. “Undesirable substances” maybe?

AlpheccaStars, now I understand the difference between the two adjectives.
So it would be correct:
They found undesired substances in the food. (already happene