...half life determines the duration of drug effect; half life = (Volume of distribution divided by clearance) x 0.693

Or

half life = Time constant x 0.693

Or

Time Constant = Volume of distribution divided by Clearance.

1 half life = 50% of the amount of drug remaining in  the body.

2 half life = 25% of the amount of drug remaining in  the body.

3 half life = 12.5% of the amount of drug remaining in  the body.

4 half life = 6.25% of the amount of drug remaining in  the body.

5 half life = 3.125% of the amount of drug remaining in  the body.

Time constant is longer than the Half life.

In one time constant decreases to 36.8% (up to less than 50% - that is to say - one time constant takes longer time than one half life)

But, if the system is about increase rather than decrease - it increases to 63.2% in one time constant.

The time constant represents the elapsed time required for the system response to decay to zero if the system had continued to decay at the INITIAL  rate, because of the progressive change in the rate of decay the response will have actually decreased in value to 36.8 % in one time constant.

It takes 3 time constants to achieve 95% of the drug concentration.

Thus 5 half life (97%) corresponds to 3 time constants (95%).

Half life:
http://sepia.unil.ch/pharmacology/?id=60

Time Constant:
https://en.m.wikipedia.org/wiki/Time_constant

FRCA equations:
http://www.frca.co.uk/documents/Equations%20for%20Primary%20FRCA.pdf

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