I would like to know if either Jennifer Love Hewitt or Neve Campbell
have done any scenes where their characters were gassed unconscious or
chloroformed. This would include television, movies, special
appearances on variety shows,guest appearances on tv shows in which
they were not regulars, etc.

My conclusion, after researching and consulting regarding this
question is that neither of these two actresses has done any scenes in
which their characters were gassed unconscious or sedated or
chloroformed. The consultations included with some devoted fans and
followers. Although I would be reluctant to post a negative finding,
the research was thorough and no further evidence or suggestions to
the contrary were made since leaving this open for further comments
for another four days.
 
A few additonal findings are: 
 
Neve Campbell did the voice of a the character Kiara in the film, "The
Lion King II - Simba's Pride, in which this lion became unconscious
during a fire.
 
Helen Mirren, playing Mrs. Tingle, in "Killing Mrs. Tingle",
interestingly was compared as an actress to both Neve Campbell and
Jennifer Love Hewitt.
(www.corona.bc.ca/films/details/killingmrstingle.html). Mrs. Tingle
was hit in the head with an arrow from a crossbow and knocked
unconscious in this film.
 
Characters played by other actors appearing in the two actresses'
films did become unconscious, for example, a killer was found
unconscious in a car in the film Scream 2; however no instances are
found in which either Jennifer Love Hewitt or Neve Campbell were
gassed unconscious or chloroformed.
 
Thank you for your question. I hope the answer is satisfactory, and
please let me know if you wish any clarification.

Not the answer I was hoping for, but I thank this person for taking
the time to honestly try and find one. Thanks for your time and hard
work.I'll have more along these lines soon.

I have researched this question and also consulted with devoted fans
of theirs of them; and no sign or recollection could be found of any
scenes in which either of these two actresses were made unconscious
with gas or chloroform. I wish to post this negative finding as my
answer.
Since the only persons if any who could answer "no" to this question
with absolute certaintly are most likely the actresses themselves, I
shall first allow other researchers the opportunity to find such a
scene, if one did take place.

A few incidental findings are:

Neve Campbell did the voice of a the character Kiara in the film, "The
Lion King II - Simba's Pride, in which this lion became unconscious
during a fire.

Helen Mirren, playing Mrs. Tingle, in "Killing Mrs. Tingle",
interestingly was compared as an actress to both Neve Campbell and
Jennifer Love Hewitt.
(www.corona.bc.ca/films/details/killingmrstingle.html). Mrs. Tingle
was hit in the head with an arrow from a crossbow and knocked
unconscious in this film.

Characters played by other actors appearing in the two actresses'
films did become unconscious, for example, a killer was found
unconscious in a car in the film Scream 2; however no instances are
found in which either Jennifer Love Hewitt or Neve Campbell were
gassed unconscious or chloroformed.

Once again, I appreciate receiving the opportunity to post and receive
payment for this answer, but let us see if other researchers can come
up with something.