What does 'non cumulative overtime' mean in this sentence?

non-cumulative overtime

This is incorrect. It should be "over time"

Here is a paraphrase

The side effect is mainly neutropenia which is reversible within 5 to 7 days, and non-cumulative as time passes.

In other words, the neutropenia does not get worse as time passes. (but I am not a doctor so I can't vouch for this)