Fantasy book about supernatural beings that can take drugs to get into a different world

I am looking for the title and author of a book I have once read in the fantasy genre.

I remember two specific elements: the first is that the protagonist, a male, is part of a group of special beings/half gods. I believe they are called the immortals or something like that. There is a fixed number of them (a hundred perhaps?) and each has superhuman abilities, but all have different ones. I believe the protagonist was very fast or could fly, but I am not certain. Now I seem to remember that sometimes people are promoted into this group, but I am not sure and how it worked, perhaps if a member died or something.

The second element I remember is that you could take a substance that transported you to another world. Like hallucinating, but the world you hallucinated was real and persistent.

A last thing, which unfortunately I am not certain of as I read many books, is that the author might not have an obviously male or female name, at least to a non-native English speaker like me.

This is the sum of my memories about the book, next to the fact that I enjoyed it. I am looking forward to your answers.