Technically speaking, if there were some other substance (i.e. the poison) in the host, the consecration would not be valid.
1. The priest who is to celebrate Mass should take every precaution to make sure that none of the things required for celebrating the Sacrament of the Eucharist is missing. A defect may occur with regard to the matter to be consecrated, with regard to the form to be observed and with regard to the consecrating minister. There is no Sacrament if any of these is missing: the proper matter, the form, including the intention, and the priestly ordination of the celebrant. If these things are present, the Sacrament is valid, no matter what else is lacking. There are other defects, however, which may involve sin or scandal, even if they do not impair the validity of the Sacrament.
2. Defects on the part of the matter may arise from some lack in the materials required. What is required is this: bread made from wheat flour, wine from grapes, and the presence of these materials before the priest at the time of the Consecration.
3. If the bread is not made of wheat flour, or if so much other grain is mixed with the wheat that it is no longer wheat bread, or if it is adulterated in some other way, there is no Sacrament.
(Pope Pius V, Papal Bull De Defectibus)
Yeah, I think I said that, or at least hinted at it. No, a communion host with poison in it is not valid matter for the sacrament.
But even if it were, the notion of substance and accidents still makes it deadly. SD