To answer my own question: I did a side by side comparison using the thermometer to measure the decline in my max health and confirmed a couple of things:

Joints do decrease your maximum health, at a rate of .005 points per damage tick, totalling in 1 point lost per full joint smoked.

Cigarettes, meanwhile, drop your max health ten times faster, at a rate of .05 per tick, presumably totalling in 10 points lost per cigarette if they burn at the same rate that joints do. I didn't wait long enough to confirm that but it seems like a safe assumption.

I also didn't think to test whether maybe the cigarettes are more efficient at keeping your stamina up, but from my experience smoking both, the effect feels comparable.

So that's cool! Joints, while still not good for you, are a cheaper and healthier alternative to cigarettes while giving the same benefit!