The '5 Half-Lives' Rule: When a Compound Is Practically Gone

If you want a single, defensible answer to “when is this compound basically out of my system?”, the rule of thumb is five half-lives. After five, roughly 97% of a single dose has been cleared, little enough that the drug is considered practically gone. To apply it to any specific compound, enter its half-life in the Peptide Half-Life Visualizer, which marks the 50%, 25%, 10%, and 1% points on the decay curve. This article shows where the rule comes from, how to use it, and why the same math, run backward, tells you when a repeated dose reaches steady state.

Where “five half-lives” comes from

The rule is nothing more than repeated halving. Each half-life removes half of whatever is left, so the eliminated fraction climbs like this:

Half-lives elapsed	Remaining	Eliminated
1	50%	50%
2	25%	75%
3	12.5%	87.5%
4	~6.25%	~93.75%
5	~3.125%	~96.9%

After five half-lives roughly 97% of a single dose is cleared, which is why ‘five half-lives’ is the standard rule of thumb for a drug being practically gone.

By around 3.3 half-lives you have cleared ~90%; by 4–5 half-lives you are at ~94–97%. Pharmacology references treat that as “effectively eliminated,” because the small remainder is below the level that meaningfully acts. There is nothing magic about the number five: it is just where the curve gets close enough to zero that the leftover no longer matters for most purposes. (For the underlying definition of half-life itself, see what a peptide’s half-life actually is.)

How to use it

Using the rule is a one-line calculation: practically gone ≈ 5 × half-life.

• Caffeine, half-life ~5 hours → ~25 hours to be practically cleared. (Worked through in how long caffeine lasts.)
• Semaglutide, half-life ~1 week → ~5 weeks. This is the washout figure in how long semaglutide and tirzepatide stay in your system, and it matches semaglutide’s own label statement that it persists for about five weeks.
• Tirzepatide, half-life ~5 days → ~25 days, or roughly four weeks.

Two honest limitations keep this from being a stopwatch. First, “practically gone” is a convention, not a hard zero: a small amount is still detectable, which matters for sensitive assays and drug testing even when it no longer has an effect. Second, half-lives are population averages; your kidney and liver function, body size, and other factors shift your personal clearance, so five half-lives is a well-grounded estimate rather than a guarantee.

The same math, run backward: steady state

The five-half-lives rule is not only about a drug leaving; it also governs a drug building up. When you dose repeatedly on a fixed schedule, each dose lands on top of the tail of the previous ones, and levels climb until the amount you take each interval equals the amount you clear. That plateau is steady state, and reaching it takes about the same 4–5 half-lives.

The symmetry is exact because it is the same exponential process viewed from two ends: elimination approaches zero on a five-half-lives timeline, and accumulation approaches its plateau on the same one.

Half-lives on a stable schedule	Approx. % of steady state reached
1	50%
2	75%
3	87.5%
4	~94%
5	~97%

Two consequences fall out of this, both counterintuitive at first:

• Time to steady state depends on half-life, not dose. A bigger dose reaches a higher plateau, but it does not get there any faster. Only the half-life sets the timeline. This is why a drug’s blood level can keep creeping up for weeks after you start, even at a fixed dose.
• For weekly GLP-1 agents, steady state takes about a month. With a roughly one-week half-life, semaglutide reaches steady state after about 4–5 weekly doses, which is one reason dose titration is spread over weeks rather than rushed. The buildup is visible in the Dose Interval Visualizer, which stacks repeated doses on the decay curve, and the clinical logic is covered in GLP-1 dosing and titration.

Why the two ends connect

It is worth seeing why washout and steady state are the same rule. Both are exponential approaches to a limit (one heading down to zero, the other up to a plateau), and an exponential covers a fixed fraction of the remaining gap each half-life (half, then half of the rest, and so on). After five steps you have covered ~97% of the distance in either direction. So the moment you know a compound’s half-life, you know two things at once: about how long until it is practically gone after stopping, and about how long until it reaches a stable level after starting. Same clock, both directions.

This is also why half-life sits underneath every dosing schedule in the field, the point developed in why peptide dosing frequency varies. One caveat carried over from half-life vs duration of action: reaching steady blood levels is not the same as reaching steady effect, since effect can lag or lead concentration.

The takeaway

Five half-lives is the workhorse rule of pharmacokinetics: after five, about 97% of a dose is cleared, so the compound is practically gone. The identical math run backward says a repeated dose reaches steady state in about the same 4–5 half-lives, and that timeline is set by the half-life alone, not by how much you take. Learn a compound’s half-life and you can estimate both its washout and its buildup in your head.

Mark the five-half-lives point for any compound in the Peptide Half-Life Visualizer, and watch accumulation to steady state in the Dose Interval Visualizer.

This article is educational information about pharmacokinetics, not medical advice. Many research peptides are not approved for human use; consult a qualified clinician before making any decision about a medication or compound.

Sources

• Elimination Half-Life of Drugs — StatPearls, NCBI Bookshelf
• Steady State Concentration — StatPearls, NCBI Bookshelf