The books make it clear that a potion does not allow the "consumer" (however that is defined) to actually cast the spell embodied in that potion, but only for the instilled spell to effect them.
So, for spells that allow for variable results, who makes that decision, and when- the consumer at the moment of consumption, or the one who brewed the potion during that creation?
Case in point-
And there are countless more, any that have a variable that "you" or "the caster" determines when the spell is cast.
While far more convenient for the consumer to have control over such, it seems that btr these must be pre-determined by the brewer at the time of creation.
Thus, the spell above would have one predermined face that it, and all potions of the same batch brewed that season, would produce, regardless of the wishes (or sex, age or etc) of the consumer.
I don't see any reason why all potions in a batch need have the same parameters in that way. It doesn't follow from the rules, AFAICS.
As to whether or not the drinker should have any input on the effects of a spell - it's a tough call. What about other spells with parameters - a levitation spell, for example? Won't the drinker be allowed to control his movement? If so, and if Disguise... does not allow control, where is the limit?
And what about charged items that are not potions? Can a person with a Disguise... wand control the transformation? If so, why not a potion, and at what point do you lose it - when the item is actually imbibed? Or as soon as it's a "consumable"?
I suggest the SG makes a decision on a case-to-case basis, after all.
The difference is that a potion doesn't allow the imbiber to cast a spell. The target of levitation can control their movements, but the caster of Disguise chooses the face.
As far as the other...
"...Once you have chosen the effect, you spend a season, add your aura to your Lab Total, and compare the total to the level of the effect. For every 5 points, or fraction thereof, by which you exceed the level, you get one dose of the potion..."
One effect, one potion, many doses of that one potion.
A spell either makes you look like X, or like Y. A potion that has X final effect is not a potion that has Y, even if the Tech/Form/Level are identical. Even if the base spell is identical- the variables differ, (and only the caster can choose those, when that one spell is cast.) To create two different effects would use the rules for creating two different potions/lab projects.
But why is drinking not casting when using other charged items is (sometimes)? Where do you draw the line?
A charged wand or earring och pair of boots allow you to cast the spell, with different effects each time. What's to say the same doesn't hold for potions and the like?
Generally speaking, items cast their own spell: "In the end, you have an item that is independent and that generates its own magical energies in order to function..." But not potions, whose description specifically states "A potion can never give anyone the ability to cast a spell..."
Think of it this way: Items mimic spellcasting, potions effectively "store" a spell already cast. (Or, at least, that is the position that sparked this thread, and one that has yet to be challenged btr.)
But I'll take the same qualified position here as in another thread (and as almost always)- that a GM always has the sovereign right to interpret the rules/world as best works for them. (The only drawback is integrating that interpretation with other published material, or with others' games, and your interpretation doesn't seem to present much of an obstacle.)
Ah, I see now; you were referring to 4th ed. rules. That's fair, I guess. I tend to disregard old editions' rules by default, but since 5th lacks any specifics regarding potions I see where you're coming from. (Or is there anything on them in Mystery Cults, perhaps?)
Anyway, it certainly makes sense that a potion-drinker has no control over the parameters of the spell cast. However, AFAICS this doesn't follow necessarily from the 5th ed. rules, and there are cases when it's less clear.
For example, potions could allow for control of their effect through a final step of adding something; I've thought of for example Potions of Homecoming, where you add a grain of soil from the place to where you wish to go (providing the arcane connection for the spell) and then drink it; or why not a Polyjuice Potion, where you add a sample of a creature whose shape you wish to assume and then drink it.
I think the above are interesting enough applications that they may be allowed for potions - after all, there's nothing in the 5th ed. rules that explicitly forbid them, and they don't violate any other rules, AFAICS.
There is nothing to this effect in mystery cults that I've seen.
I don't believe that we'll see any rules that restrict control of charged items if and only if they happen to be created as potions in any future book. Such rules would limit what characters can do just for the sake of adding more complexity. Sounds like a loose loose situation to me.