So, I got the Purity perk, and wanted to make some ravage health potions. I opened the lab, selected "Ravage Health" from the list of effects, then selected two ingredients, I think it was saber cat eye and spadetail, maybe - one of the fish. Anyhow, it was trying to make a potion with another affect that both of them shared, instead of a poison of ravage health.

Is there any way to force it to use a specific effect? Or is it just hard coded that certain combinations produce a potion or a poison when Purity is in effect? I would have thought Purity would give you a choice of poison or potion.

The effect priorities are based on money: if the most expensive single effect is a Potion, you get a Potion. If it's a Poison, you get a Poison. Purity negates all Poison riders in Potions, and vice versa.

Saber Cat Eye + Silverside Perch normally creates a Potion of Restore Stamina with a Ravage Health rider. This is because the base value of Restore Stamina is 25, and Ravage Health is a measly 6 - thus, Purity will always choose Restore Stamina for that combo, and toss Ravage Health.

Try Saber Cat Eye + Silverside Perch + Juniper Berry (or Skeever Tail.)