The relation of a schizophrenia drug side effect and caffeine intake
I used Google Scholar with the search terms of clozapine, caffeine.
I only found small scale studies, but they agree that it is likely that Clozapine levels are affected by caffeine intake.
To really understand the implications of this it would be necessary to research what the range of Clozapine levels actually are in patients and what the effective level is, as well as the levels that give side-effects.
Only then can the changes due to caffeine be seen to be significant or not
Example paper studying 12 volunteers below (full text available)
Hägg, S., Spigset, O., Mjörndal, T. and Dahlqvist, R. (2000), Effect of caffeine on clozapine pharmacokinetics in healthy volunteers. British Journal of Clinical Pharmacology, 49: 59-63.
https://doi.org/10.1046/j.1365-2125.2000.00111.x
Available at:
https://bpspubs.onlinelibrary.wiley.com/doi/full/10.1046/j.1365-2125.2000.00111.x