Is an allergic reaction possible with probiotics such as florajen3? i've had l. acidophilus in yogurt, but i'm worried about b. lactis/longum.

Improbable: I have never heard of a case of probiotic allergy. Some people may have more bowel movements from taking it however but that's not an allergic reaction.

Yes! EvenWithNothing: Reactions, allergic & otherwise, are always under the internal control of the individual exhibiting the reaction & theories as to what may have triggered the behavior (reaction) are always just that, theories, guesses, rarely with significant scientifically verified evidence of accuracy, no matter the degree of the person guessing. Why? Control and complexity are within the individual reacting.