Interaction of chemicals 1) most commonly observed when two chemicals are given together occurs when the COMBINED EFFECT OF TWO CHEMICALS IS EQUAL TO THE SUM THE EFFECTS OF EACH AGENT GIVEN ALONE 1+1= 2 2) occurs when the combined effects of two chemicals are MUCH GREATER THAN THE SUM OF THE EFFECTS of each agent given alone 1+1=3 PRODUCES MUCH GREATER EFFECTS