T Nation

Half Life

I’ve gotten curious about this…

Lets assume a 10 day “half-life” for absorption. So in 10 days, you absorb 1/2 the initial dose, or do you?

Assume 100mg injection.

Day 1: absorb 100 mg *1/2 /10 = 5 mg
Day 2: is it 5 mg, or (100-5)mg * 1/2 / 10 = 4.75 mg
Day 3: is it 5 mg, or (100 - 5 - 4.75)mg * (1/2) / 10 = 4.5125 mg

I am thinking the calculation is correct, since day 2 is like starting day 1 with 95 mg.

If by absorb you mean lose, then

Day 1: Lose 50 mg. 50 mg remains in system.
Day 2: Lose 25 mg from the 50. 25 mg still remains
Day 3 Lose 12.5 mg (12. 5 mg remains.)
Day 4 Lose about 6 mg (again about the same remains.)

Etc.

Assuming the exponential equation is exactly met, which is never the case but is “close enouogh for government work” in very many cases.

Bill,

Wouldn’t it be 50mg lost and 50mg remaining after day 10, not day 1, if the half life were 10 days???

…from an on-line calculator. If you were to inject 100mg of TestCyp (with a 7day half-life) then on day 8, so seven days later, you would/should still have 50mg in your system (see below)

Day 1: 100mg T, 0mg E/D/TInjection Day
Day 2: 90.6mg T, 0mg E/D/T
Day 3: 82mg T, 0mg E/D/T
Day 4: 74.3mg T, 0mg E/D/T
Day 5: 67.3mg T, 0mg E/D/T
Day 6: 61mg T, 0mg E/D/T
Day 7: 55.2mg T, 0mg E/D/T
Day 8: 50mg T, 0mg E/D/T
Day 9: 45.3mg T, 0mg E/D/T
Day 10: 41mg T, 0mg E/D/T
Day 11: 37.1mg T, 0mg E/D/T
Day 12: 33.6mg T, 0mg E/D/T
Day 13: 30.5mg T, 0mg E/D/T
Day 14: 27.6mg T, 0mg E/D/T
Day 15: 25mg T, 0mg E/D/T
Day 16: 22.6mg T, 0mg E/D/T
Day 17: 20.5mg T, 0mg E/D/T
Day 18: 18.6mg T, 0mg E/D/T
Day 19: 16.8mg T, 0mg E/D/T
Day 20: 15.2mg T, 0mg E/D/T
Day 21: 13.8mg T, 0mg E/D/T
Day 22: 12.5mg T, 0mg E/D/T
Day 23: 11.3mg T, 0mg E/D/T
Day 24: 10.3mg T, 0mg E/D/T
Day 25: 9.3mg T, 0mg E/D/T
Day 26: 8.4mg T, 0mg E/D/T
Day 27: 7.6mg T, 0mg E/D/T
Day 28: 6.9mg T, 0mg E/D/T
Day 29: 6.3mg T, 0mg E/D/T
Day 30: 5.7mg T, 0mg E/D/T

Kinda my point. Should I trust the calculator?

7 day half life…

day 1: 100mg
day 2: should it be 100 - (100/2 * 7) = 92.9 mg left?

…but you’re assuming that it’s a linear loss with that formula are you not?

  • edit * no, I guess your not are you

Just remember that this is theoretical. Your particular body and metabolism will alter the “half-life” somewhat.

BD

[quote]Babo wrote:
Bill,

Wouldn’t it be 50mg lost and 50mg remaining after day 10, not day 1, if the half life were 10 days???[/quote]

Yes, I used a 1 day half life example, forgetting that you had stipulated 10 days, and forgetting to say that I was referring to a 1 day half-life. Oops.

[quote]hebsie wrote:
…but you’re assuming that it’s a linear loss with that formula are you not?

  • edit * no, I guess your not are you[/quote]

Half-life estimates for drugs do not follow a linear loss, but a logarithmic curve; i.e., a percentage is lost with each unit of time.

For the ultra-particular the equations are here:

But that is only a reasonable guess. Testosterone follows a multi-compartment model–from injection to serum, partition between free and bound testosterones, distribution in tissues, metabolism at a few sites.

So the half-lives you see reported for the various preparations are empirically derived.