1) Act reciprocally 2) Affect mutually 3) Communicate directly 4) Converse 5) Create synergy 6) Have reciprocal effect 7) Influence reciprocally 8) Mix with each other 9) Move 10) To act one upon another 11) To act upon each other 12) To affect each other 13) Work together 1) Assort 2) Collude 3) Foregather 4) Forgather 5) Intervene 6) Marginalise 7) Marginalize 8) Mingle 9) Philander 10) Relate 11) Socialise 12) Socialize 13) Transact 14) Womanise 15) Womanize 
• (v. i.) To act upon each other; as, two agents mutually interact. • (n.) A short act or piece between others, as in a play; an interlude; hence, intermediate employment or time.
Interaction between pairs of drugs and/or medicines may occur, meaning one does not produce the desired effects when in the presence of the other, and/or they have undesirable side effects when taken together.
In`ter·act' noun [ Prefix inter- + act . Confer Entr'acte .] A short act or piece between others, as in a play; an interlude; hence, intermediate employment or time. Chesterfield. 
[v] - act together or towards others or with others
two or more things act upon each other
when one thing acts upon another
verb act together or towards others or with others; `He should interact more with his colleagues`
to act one upon another.
do something together or with others