It is a game of chance. There are multiple examples of proteins that have a Chirality. While being made of the same atoms (and even of the same bonds), they are not identical and may have different properties.
While I'm certainly no expert on this topic, it is easily possible that a totally harmless
L-Protein could be a very poisonous
In other cases it can simply be useless to us, as our enzyms can't process it (the form of a molecule is as important as the composition to our body).
Why exactly most proteins on earth are
L has apparently not been explained completely and satisfactory, it is entirely possible that the whole alien ecosystem is based on
Regarding psychological effects that is up to your imagination. There are so many different substances which can influence the human (or, broader, the terrestrial) mind that you can possibly get any effect that can be achieved by drugs as well. Likewise with physiological effects.