Life Elixer, how does it work?

In my novel I want to use an elixer of Life, however I don't want it to make my character to live forever, just be ever living, if that makes sense. He ages, just very slowly. It doesn't make him immortal or unkillable, he can die just as any of us can.

Anyone have ideas on how this might be possible?

Well, it can make him effectively immortal, but as it wears down he slowly ages.

As far as the actual function of it, well, it may reduce cellular decay, and cause his body to run extremely efficiently. All in all, he'd be very healthy for the duration, less likely to get sick, recover reasonably quickly from injury, process and store food better, think clearer. And slow the aging process right down. But as it wears off, the aging slowly accelerates until that of a normal human being. So the end of his very long life will be more or less the same as everyone else.

He'll just take a long time getting there.

Are there any side effects of this? Would toxins take longer to work their way out of him? Or would they be processed and expelled faster?

Is his blood still coursing through with the elixir, and if he loses blood does his aging accelerate? Could it be used to help heal others?

The above questions are very important. One theory seen at the "Answers In Genesis" site as to why people lived almost a thousand years at times before the Floord was a protective layer of water around the earth blocking the sun's harmful rays. So, perhaps this elixir gets into the bloodstreama nd winds up creating a similar layer around the skin itself. One's skin might feel a little different or not,, but in the end, somehow those harmful rays just bounce off the person. Or, are converted into something that is not harmful, maybe slowed down so they go from TV light wvelength to normal visible light wavelength would react when it its the skin, just nothing.

(Note, I am not a scientist, I don't claim that it is actually possible for skin to somehow react to alter the way wavelengths hit the skin. But you have to admire my creativity. :-) )

Sounds to me like you might not want the Elixer of life, but rather the old alchemy "panacea" which was suppose to cure all sickness, and prolong life, but did not give one immortality.

Then there is the dark magic kind of life prolonging spells where one took the life force of another and extended their life by the length/strength of the life force one took?

Improve cellular regeneration. At the moment, our repair mechanisms tend to not make perfect repairs, so over time everything wears out. Once you get past that barrier, you may have to worry about the telomerase chain shortening problem.