Potion Miscibility

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Use of Magic Items
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DMG

The magical mixtures and compounds which comprise potions are not
always compatible. You must test the miscibility of potions whenever:

    1) two potions are actually intermingled, or

    2) a potion is consumed by a creature while another such liquid already consumed is still in effect

While it is possible to prepare a matrix which lists each potion type and
cross references each to show a certain result when one is intermingled
with the other, such a graph has two drawbacks. First, it does not allow for
differences in formulae from alchemist and/or magic-user. Second, it will
require continual addition as new potion types are added to the campaign.
Therefore, it is suggested that the following table be used -- with,
perhaps, the decision that a delusion potion will mix with anything,
that oil of slipperiness taken with oil of etherealness will always increase the
chance for the imbiber to be lost in the Ethereal Plane for 5-30 days to 50%,
and treasure finding mixed with any other type of potion will always yield
a lethal poison. Whatever certain results you settle upon far your campaign,
the random results from the table apply to all other cases.
 

    POTION MISCIBILITY TABLE
Dice Score Result
01 EXPLOSION
Internal damage is 6-60 h.p.
those within a 5 radius take 1-10 h.p. if mixed externally, 
all in a 10 radius take 4-24 hit points, no save. 
02-03 Lethal poison
results, and imbiber is dead; 
if externally mixed, 
    a poison gas cloud of 10 diameter results, 
        and all within it must save versus poison or die. 
04-08 Mild poison
which causes nausea and loss of 1 point each 
of strength and dexterity for 5-20 rounds, 
no saving throw possible;
one potion is cancelled, 
the other is at half strength and duration. 
(Use random determination for which is cancelled and which is at half efficacy.) 
09-15 Immiscible
Both potions totally destroyed, 
as one cancelled the other. 
16-25 Immiscible
One potion cancelled, 
but the other remains 
26-35 Immiscible
result which causes both potions to be at half normal efficacy when consumed. 
36-90 Miscible
Potions work normally unless their effects are contradictory, 
    e.g. diminution and growth, which will simply cancel each other. 
91-99 Compatible
result which causes one potion 
    (randomly determined) 
to have 150% normal efficacy. 
    (You must determine if both effect and duration are permissible, 
         or if only the duration should be extended.) 
00 DISCOVERY
The admixture of the two potions has caused a special formula which will cause one of the two potions only to function, 
but its effects will be permanent upon the imbiber. 
(Note that some harmful side effects could well result from this. . . )

Roll for miscibility secretly whenever it occurs.
Give no uncalled-for clues until necessary.
 

Q: If a character mixes two potions
and rolls a 00 on the DMG?s potion
miscibility table (page 119), can he
duplicate the resulting potion if he
knows the exact formulas of the two
original potions?
A: No. Potions are manufactured through
the art of alchemy, not the science of
chemistry. When two completed potions
are combined, there is no way to predict
the result; if the two original potions are
duplicated and mixed again, a second
miscibility roll must be made.
(147.10)