There is a wide variety of opinion on Vit C. I think I can see where the DR was going with his/her statement ....Yet, maybe a poor choice of words.
The body does not become IMMUNE to Vit C.....After a certain amount of intake the body will reduce its absorption and the Vit C will be passed to waste.....(this can also be beneficial....it changes the PH of the Urinary tract which can help fight UTI's).....Research shows that most individuals can tolerate up to 4000mg of Vit C before there is any adverse reaction...which is usually diarrhea......There can be some 'rebound' effect coming off high doses quickly, if you choose to do so.
The 'sense of well being' is generated from the other ingredients in the packet....namely the Magnesium which has a calming effect......and the B Vits which help with stress related responses....
Emergen C is designed as a daily supplement........and IMO can be used according...
THere is a lot of reading on the subject which may be helpful.
Even the Emergen C site has info:http://www.alacer.com/info.asp
I am in no way affiliated....but I am a consumer.