Currently a lot of damaging chems seem to be just that, chems that damage you. And while that can be fine with some of them, it does kind of get boring when what changes between one chem and another is what kind of damage it causes and how fast it goes away. Besides that, per tick constant damage numbers do run into the issues of stacking, and the current solution of limiting the amount of metabolized chems at any moment does feel a bit underwhelming.
As for that, what i have to suggest is what a few chems already do, and what a lot of ss13 servers use to remedy this without needing to limit metabolization stacking: Staging effects and adding some extra “flavor only” effects to chems as well.
A way to exemplify this, would be the difference of a chem that deals 2 tox per cycle, and one that makes you shiver/shudder and walk in random directions for 10 cycles, then start dealing 2 tox ontop of that, or that has a chance per cycle of giving you a slowdown, and things of the sort. With those sorts of effects, stacking loses a bit of it’s punch, since two chems that each need 15 seconds to pick up speed aren’t really that much better together, and the variance in timings and add-on effects can help keep them unique from one another, as well as giving the affected player hints as to what’s going on before things go bad.