The use of the word "entheogen" in its broad sense as a synonym for "hallucinogenic drug" has attracted criticism on three grounds:

On pragmatic grounds, the objection has been raised that the meaning of the strict sense of "entheogen", which is of specific value in discussing traditional, historical and mythological uses of entheogens in religious settings, is likely to be diluted by widespread, casual use of the term in the broader sense.
Secondly, some people object to the misuse of the root theos (god in ancient Greek) in the description of the use of hallucinogenic drugs in a non-religious context, and coupled with the climate of religious tolerance or pluralism that prevails in many present-day societies, the use of the root theos in a term describing non-religious drug use has also been criticised as a form of taboo deformation.
Thirdly, there are some substances that at least partially fulfill the definition of an entheogen that is given above, but are not considered hallucinogenic in the usual sense. One important example is the bread and wine of the Christian (especially Roman Catholic and Episcopal) Eucharist -- assuming that the Eucharist was always non-entheogenic as it has been in most modern-era Christian groups. The 'bread' and 'wine' of early Christianity were discussed and treated in a way that fits the description of an entheogenic substance; ingesting the Eucharist induced the Holy Spirit, as a shared unity-experience in the mystic altered state.