I have verified the lifespan loss for Vitamin C and Vitamin D is determined every time that the effect of it is granted, not when the drug is given to one's monster.
I had been raising a dragon along a modified "Shock Treatment" style. I had fed him a Vitamin D, worked him only on week 4, fed meat every time, along with abundant mint/taffy use.
My monster still aged, and I received retirement warnings and he eventually died of old age.
I had saved on the 1st week of January, if I rested him (with taffy and mint leaf) he would die on the 4th week of January (3 weeks of life left).
If worked in a light drill, he dies on the 2nd week of January (1 week of work + 2 weeks of Vit D life loss).
I then fed him Vit C and rested him, he lived until the 4th week.
Given Vit C and worked, he would die if rested the following week (1 week work + 1 week Vit C + 1 week rest).
As a control, I fed him one of the other drugs available at the shop and rested him, he died on the 2nd week (1 or 2 months life loss at the time of the drug being given and stats are adjusted).

But on a side note I did find out that if you send a monster on expedition it WILL live past it's normal expiration date and you can freeze and save your monster after he returns (I had to find something to do with my dragon, being affected by VitD and having 3 weeks to live, he couldn't work or fight any more, so I sent him on expedition to clear out some obstacles)

This is given just as a word of caution. If you're going to try the Shock Treatment to get a monster to live forever, don't be impatient, as I have found out that it can cost the monster it's life.
(1 week work + 2 weeks Vit D - 1 week meat = 2 weeks aged for every week alive in this manner)

Happy Ranching "

Thank you! I just finished off my Golem and am taking a few days off from research. This is very good to know for certain, I had suspected it earlier with short-lived monsters I had on VitD their whole lives, but had not proven it exactly.

just took a look at your golem, lisa! quite impressive...a 17 yr 11 mo old golem with all 999s and all techs.

anyone who wants to see it should go to the raising methods part of the mr1 archive, and choose the "shock treatment" thread.