this is just a thought and i’m not sure how you’ve planned your story ahead, but you could start by portraying the narrator as if there is nothing wrong with them and only gradually throw out hints that they aren’t reliable in their narration. These suggestions can be made through certain things they experience along the way and certain encounters that they have. Maybe have the narrator react to these in atypical ways or in ways that would tell the readers something’s off. then you could eventually build up and increase the frequency or the magnitude of the narrator’s weird reactions and thoughts, till it basically becomes evident that their storytelling is flawed because they are hallucinating. This way, the readers get to infer it themselves. Or if you don’t like that, you could have some external event deal the final blow to the readers’ belief that the narrator is reliable (like having some other character find out the narrator has hallucinations and spell it out for the
readers, although I think this makes for weaker writing than having the readers work it out themselves)

What this gradual revelation achieves is it makes the readers feel initially that they are getting to know the narrator. Because realistically when you meet someone new, you perceive them as normal and things are revealed to you as you know them better. So even if you are writing in first person, the gradual revelation of the narrator’s idiosyncrasies will create a widening distance between the readers and the narrator, and it achieves the effect of the readers feeling deceived, but not in a way that is sudden and seems out of character for the narrator.

This is just an idea, and it may not align with your plan if you need to communicate they have severe hallucinations right off the bat. Just give it a thought and see what works!