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Functional calculator

Half-life calculator.

When does a drug reach steady state? How long after the last dose until it's effectively cleared? The same arithmetic that's behind every titration schedule and every washout instruction — exposed.

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The drug

Most peptide & drug half-lives are published — see the reference profiles
168 = weekly · 24 = daily · 72 = every 3 days

Steady state & washout

~34 d
Time to ~95% steady state (≈5 half-lives)
Steady-state peak (Cmax,ss) mg equiv.
Steady-state trough (Cmin,ss) mg equiv.
Accumulation ratio
Time to washout (5% remaining)
Time to complete clearance (~1%)
Half-lives elapsed at steady state:
1 → 50% · 2 → 75% · 3 → 87.5% · 4 → 93.75% · 5 → 96.875%
The math:
Time to steady state ≈ 5 × t½
Accumulation ratio R = 1 / (1 − e−kτ)  where k = ln(2)/t½, τ = dosing interval
Cmax,ss = Dose × R / (1 − e−kτ) · Cmin,ss = Cmax,ss × e−kτ
Washout to f of peak: t = t½ × log₂(1/f)
Methodology

When this number matters.

Titration is half-life math.

The semaglutide 4-week dose-escalation interval is approximately 5 half-lives — long enough for the new dose to reach steady state before the next bump. Faster titration outruns clearance and stacks side effects.

Washout matters for pregnancy.

FDA guidance on semaglutide recommends discontinuation ≥2 months before planned conception. That's roughly 9 half-lives — well past the 5% washout threshold. Long-half-life drugs are unforgiving here.

Pulsatile peptides are different.

Short-half-life GH-releasers (Ipamorelin, GHRP-2) are dosed multiple times daily because the pulse — not the steady level — is therapeutic. Steady-state arithmetic doesn't fully describe them. Background on peptide pharmacology.

Linear approximation

This uses linear (first-order) pharmacokinetics. Most drugs and peptides at therapeutic doses follow this. Saturable metabolism, deep-compartment redistribution, and protein-binding effects can deviate from the model — confirm with the drug's published PK data for high-stakes decisions.

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