Suicide is always an option. In fact, Omega already presented you with it as an option, the consequences for not choosing. If you would in general carry around such a poison with you, and inject it specifically in response to just such a problem, then Omega would already know about that, and the information it offers would take that into account. Omega is not going to give you the opportunity to go home and fetch your poison before choosing a box, though.

EDIT: That said, I find it puzzling that you'd feel the need to poison yourself before choosing the falsehood, which has already been demonstrated to have positive consequences for you. Personally, I find it far easier to visualize a truth so terrible that it leaves suicide the preferable option.