How did you handle that sort of thing? I don't recall tables for it, but again I didn't play 1e and barely play Basic so maybe they had rules for it there.
1e DMG page 119
POTION MISCIBILITY
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 for 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 normal (random selection).
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.