How do mutagenic serums work?

I remember awhile back it was dependant on health, but in my last few games I’ve rarely been getting more than one mutation per serum use-- which means it’s cheaper to just use regular mutagen.

Did something change?

Most serums have 1 guaranteed mutation, 66% for another and 50% for yet another one (rolled independently). The “rare” ones and medical have higher chances, meaning it’s worth concentrating them. Untargeted has 3 guaranteed, then 75%/66%/66%.
Targeted mutagens always have 1. Untargeted has 100%/66%/50%.

Clear win for serums.

That said, you take fatigue/hunger/thirst/pain hits per mutation, too. Serums should not be immediately injected if found.