Current U.S. sildenafil labeling reports a terminal half-life of about 4 hours for parent sildenafil. Its major circulating active N-desmethyl metabolite also has a terminal half-life of about 4 hours.
That value describes the late pharmacokinetic decline in plasma concentration. It does not mean that exactly half of the administered dose remains in the body four hours after swallowing a tablet, nor does it mean that sildenafil's clinical effect stops precisely at four hours.
This page explains how the approximately 4-hour terminal half-life is derived and how it relates to clearance, distribution and concentration decline. Questions about how long an erectile response can remain enhanced belong on the sildenafil duration page.
Sildenafil half-life is the time associated with a 50% reduction in plasma concentration during the relevant elimination phase. Current labeling summarizes the terminal half-life of sildenafil as approximately 4 hours.
The word terminal matters because the value is derived from the late declining portion of the concentration-time profile rather than from the entire period immediately after administration. Earlier concentrations are simultaneously influenced by absorption and distribution.
Within the broader sildenafil pharmacokinetic framework, half-life therefore describes the time scale of terminal decline rather than peak concentration, total exposure or duration of clinical effect.
| Concept | Sildenafil-Specific Meaning |
|---|---|
| Terminal half-life | Approximately 4 hours for parent sildenafil. |
| Active-metabolite terminal half-life | Approximately 4 hours for the major N-desmethyl metabolite. |
| Primary domain | Late plasma concentration decline. |
| Not equivalent to | Effect duration, Cmax, AUC or time until absolutely no drug remains. |
Half-life describes repeated proportional decline. In an idealized terminal-phase model with a 4-hour half-life, the concentration present at the start of that terminal interval would fall to about 50% after 4 hours, 25% after 8 hours and 12.5% after 12 hours.
Those percentages refer to the modeled concentration relative to the starting point of the terminal-phase calculation. They should not be interpreted as percentages of the original oral dose remaining in the body, because absorption, distribution, metabolism and excretion have already altered the relationship between dose and plasma concentration.
After about five half-lives, roughly 3% of the terminal-phase starting concentration would remain mathematically. This is a pharmacokinetic approximation rather than a precise statement about detectability or clinical activity.
| Approximate Elapsed Time | Elapsed 4-Hour Half-Lives | Conceptual Terminal-Phase Fraction Remaining |
|---|---|---|
| 0 h | 0 | 100% |
| ~4 h | 1 | 50% |
| ~8 h | 2 | 25% |
| ~12 h | 3 | 12.5% |
| ~16 h | 4 | 6.25% |
| ~20 h | 5 | 3.125% |
Terminal half-life is calculated from the late approximately log-linear decline in sildenafil plasma concentration after the earlier absorption and distribution phases have become less dominant.
Researchers estimate the terminal elimination-rate constant, commonly written as lambda-z or λz, from serial concentration measurements. Terminal half-life is then calculated as ln(2) divided by λz.
Current sildenafil labeling reports a terminal half-life of about 4 hours. The complete shape of the earlier and later plasma profile is examined on the sildenafil concentration-time page.
| Curve Phase | Primary Influence |
|---|---|
| Absorption phase | Systemic input produces rising concentrations. |
| Distribution / early decline | Movement between plasma and tissues contributes to changing concentration. |
| Terminal phase | Late approximately log-linear decline used to estimate λz. |
| Terminal half-life | ln(2) / λz; approximately 4 hours for sildenafil. |
Clearance and half-life are related but answer different questions. Clearance describes how efficiently sildenafil is removed from the systemic drug pool, whereas half-life describes how quickly plasma concentration declines.
In a simplified one-compartment model, half-life is related to distribution volume and clearance by the relationship t½ ≈ 0.693 × V / CL. This is useful for understanding direction: lower clearance tends to lengthen the decline time scale when distribution is otherwise unchanged.
Sildenafil disposition is not adequately summarized by blindly inserting any reported distribution and clearance values into that simple equation. Terminal half-life is derived from the observed terminal slope, and the distribution parameter governing that terminal phase is not necessarily the same as the reported steady-state volume of distribution. The removal-efficiency concept remains on the sildenafil clearance page.
| Feature | Half-Life | Clearance |
|---|---|---|
| Primary dimension | Time | Volume per time |
| Main question | How quickly does concentration decline? | How efficiently is parent sildenafil removed? |
| Affected by distribution? | Yes | Defined separately from distribution |
| Lower clearance, all else equal | Tends to lengthen half-life | Means slower removal efficiency |
| Same concept? | No | No |
Half-life depends on distribution as well as elimination because plasma concentration reflects only the circulating component of a drug that also partitions into tissues.
Current sildenafil labeling reports a mean steady-state volume of distribution, Vss, of approximately 105 L, indicating distribution beyond the vascular plasma compartment. Sildenafil and its major active metabolite are also approximately 96% bound to plasma proteins.
Those values provide distribution context but do not directly calculate the approximately 4-hour terminal half-life. Vss describes steady-state distribution behavior, whereas terminal half-life is derived from the late concentration-time slope. The distribution parameters themselves are covered on the sildenafil distribution page.
| PK Parameter | Sildenafil Context | Half-Life Relevance |
|---|---|---|
| Vss | Approximately 105 L | Shows substantial distribution beyond plasma. |
| Plasma protein binding | Approximately 96% | Part of the distribution environment but not a direct half-life calculation. |
| Terminal slope | Estimated from late concentration data | Direct basis for terminal half-life. |
| Terminal half-life | Approximately 4 h | Reflects combined disposition behavior. |
A 4-hour half-life does not identify a specific sildenafil concentration at 4 hours. The absolute concentration depends on the concentration from which the terminal decline began and on the earlier dose, absorption and distribution history.
Two sildenafil profiles can therefore share a similar terminal half-life while having different Cmax values or AUC. Current formulations can also differ in their early absorption profile without necessarily having a different labeled terminal half-life.
For example, current conventional tablet and VYBRIQUE oral-film labeling both describe sildenafil and its active metabolite as having terminal half-lives of about 4 hours even though their absorption and food-effect profiles are not identical. Peak concentration remains on the sildenafil Cmax page and integrated exposure on the sildenafil AUC page.
| Metric | What It Describes |
|---|---|
| Terminal half-life | Late proportional concentration-decline time scale. |
| Cmax | Highest observed plasma concentration. |
| AUC | Integrated systemic exposure across time. |
| Tmax | Time at which the observed peak concentration occurs. |
AUC and half-life describe different properties of sildenafil pharmacokinetics. AUC integrates concentration over time, while half-life characterizes the relative rate of late concentration decline.
An interaction or physiological condition can substantially increase sildenafil AUC by reducing clearance or increasing systemic exposure without allowing the resulting fold-change in AUC to be translated directly into an identical fold-change in half-life.
Conversely, knowing that sildenafil has an approximately 4-hour terminal half-life does not reveal the amount of systemic exposure generated by a particular dose. AUC remains the appropriate metric for that question.
| Question | Half-Life | AUC |
|---|---|---|
| Describes time scale of terminal decline? | Yes | No |
| Measures integrated systemic exposure? | No | Yes |
| Reported sildenafil reference | About 4 hours | Depends on dose and study conditions |
| Can one be calculated directly from the other alone? | No | No |
The approximately 4-hour terminal half-life is a pharmacokinetic parameter. Sildenafil duration is a pharmacodynamic and clinical concept describing how long enhanced responsiveness can remain observable.
Current ED labeling demonstrates erectile response for up to 4 hours after dosing, with the response at 4 hours diminished compared with the response at 2 hours. The similarity between that time scale and the approximately 4-hour terminal half-life should not be interpreted as proof that one directly determines the other.
An effect can persist while concentration is declining, and measurable sildenafil can remain after the clinically relevant response has weakened. The duration evidence belongs on the sildenafil duration page.
| Concept | Domain | Sildenafil Context |
|---|---|---|
| Terminal half-life | Pharmacokinetics | About 4 hours |
| Erectile-response duration | Pharmacodynamics / clinical observation | Response demonstrated for up to 4 hours, diminished at the later time point |
| Same measurement? | No | No |
| Direct one-to-one relationship? | No | Depends on the concentration-effect relationship |
The phrase 'stays in the system' does not have one precise pharmacokinetic definition. A terminal half-life of about 4 hours means concentrations decline progressively rather than sildenafil disappearing at a fixed clock time.
Using a simple repeated-halving approximation, roughly 3% of the terminal-phase starting concentration would remain after five 4-hour half-lives, or about 20 hours. Even then, this does not mean every assay would report the same result or that no sildenafil-related material could be measured.
Detectability depends on the analytical method, specimen, sampling time and whether parent sildenafil or metabolites are being measured. Residual detectability also should not be equated with clinically meaningful pharmacological activity.
| Question | What the ~4-Hour Half-Life Can Tell You |
|---|---|
| How fast does terminal plasma concentration decline? | Directly relevant. |
| When is absolutely no sildenafil left? | Not defined by a single half-life value. |
| What remains after ~20 hours in a simple terminal model? | About 3% of the terminal-phase starting concentration. |
| How long can sildenafil be analytically detected? | Depends on assay and sampling conditions. |
| How long does the clinical effect last? | Cannot be determined from half-life alone. |
Sildenafil half-life is estimated from serial plasma measurements collected across the concentration-time profile. Researchers identify a set of late concentration points that approximate a log-linear terminal decline.
A regression of the logarithm of concentration against time provides the terminal slope, from which the terminal elimination-rate constant λz is obtained. Half-life is then calculated using t½ = ln(2) / λz.
The estimate therefore depends on the quality and duration of late sampling. If the terminal phase is sampled too sparsely or for too short a period, the estimated λz and half-life can be less precise.
| Study Element | Role in Half-Life Estimation |
|---|---|
| Serial plasma samples | Define the concentration-time trajectory. |
| Late concentration points | Identify the terminal approximately log-linear region. |
| λz | Terminal elimination-rate constant estimated from the late slope. |
| Calculation | t½ = ln(2) / λz |
| Reported sildenafil result | Terminal half-life about 4 hours. |
The approximately 4-hour value is a population-level pharmacokinetic summary rather than an invariant biological constant for every individual. Observed terminal slopes can vary because distribution, metabolism, clearance and measurement conditions vary.
Sildenafil is cleared predominantly through hepatic metabolism, principally CYP3A4 with a smaller CYP2C9 contribution. Factors that reduce metabolic clearance can increase sildenafil exposure and can also alter the late concentration-time profile from which terminal half-life is estimated.
The magnitude of a change in AUC or Cmax should not, however, be converted automatically into a proportional change in half-life. Broader sources of variability are covered on the sildenafil PK variability page.
| Source of Variation | Possible Relationship to Half-Life |
|---|---|
| Metabolic clearance | Changes the rate at which parent sildenafil is removed. |
| Distribution | Changes the relationship between body drug amount and plasma concentration. |
| CYP inhibition or induction | Can alter the elimination component of the profile. |
| Physiological differences | Can alter PK behavior between populations. |
| Terminal sampling | Can affect precision of the estimated λz. |
Current sildenafil labeling provides strong evidence that organ function can change clearance and exposure, but it does not justify assigning one new universal half-life value to every renal- or hepatic-impairment group.
In severe renal impairment with creatinine clearance below 30 mL/min, sildenafil clearance was reduced and parent sildenafil AUC and Cmax were approximately doubled. In mild-to-moderate hepatic impairment, AUC increased approximately 85% and Cmax approximately 47% because clearance was reduced.
Those findings demonstrate altered disposition, but an AUC increase is not itself a half-life measurement. Population-specific PK interpretation remains on the hepatic impairment pharmacokinetics and renal impairment pharmacokinetics pages.
| Population Context | Measured Sildenafil Finding | Half-Life Interpretation |
|---|---|---|
| Severe renal impairment | Parent AUC and Cmax approximately doubled | Shows altered disposition but does not by itself define a new half-life. |
| Child-Pugh A/B hepatic impairment | AUC approximately +85%; Cmax approximately +47% | Reduced clearance is demonstrated, but terminal half-life requires direct slope data. |
| Severe hepatic impairment | Not adequately studied in current labeling | Do not infer a terminal half-life value. |
Sildenafil's major circulating metabolite is N-desmethyl sildenafil. It is formed through metabolism of parent sildenafil, remains pharmacologically active and has its own concentration-time profile.
Current labeling reports that both parent sildenafil and the active metabolite have terminal half-lives of about 4 hours. The similarity in terminal half-life does not mean their concentration profiles or pharmacological contributions are identical.
The metabolite has approximately 50% of the parent's in vitro PDE5 potency and reaches plasma concentrations around 40% of parent sildenafil in the referenced healthy-volunteer context. Its detailed exposure and pharmacological contribution remain on the sildenafil active metabolite page.
| Analyte | Terminal Half-Life | Additional Context |
|---|---|---|
| Parent sildenafil | About 4 hours | Primary circulating parent drug |
| N-desmethyl sildenafil | About 4 hours | Major circulating active metabolite |
| Metabolite PDE5 potency | Not a half-life parameter | Approximately 50% of parent in vitro |
| Metabolite plasma concentration | Not a half-life parameter | Approximately 40% of parent in the referenced healthy-volunteer context |
The clearest sildenafil-specific summary is that parent sildenafil and its major active metabolite each have terminal half-lives of approximately 4 hours in current labeling. That figure describes the late concentration-decline time scale, not peak exposure or duration of clinical effect.
Half-life belongs within a larger disposition system. Sildenafil has a reported Vss of approximately 105 L, is cleared predominantly through CYP3A4-mediated metabolism with a smaller CYP2C9 contribution and shows concentration profiles whose magnitude can change substantially when clearance changes.
Half-life should therefore be interpreted alongside, rather than substituted for, clearance, distribution, AUC and Cmax. For the full sequence, the sildenafil pharmacokinetics hub connects these parameters with absorption, metabolism and systemic exposure.
| Research Question | Sildenafil-Specific Answer |
|---|---|
| What is sildenafil's terminal half-life? | About 4 hours. |
| What is the active metabolite's terminal half-life? | About 4 hours. |
| How is terminal half-life estimated? | From the late log-linear concentration slope using λz. |
| Does 4 hours mean sildenafil is gone after 4 hours? | No. |
| Does 4 hours mean the clinical effect lasts exactly 4 hours? | No. |
| What other PK parameters provide context? | Clearance, distribution volume, AUC, Cmax and the full concentration-time profile. |
Current U.S. sildenafil labeling reports a terminal half-life of about 4 hours for parent sildenafil.
The major circulating N-desmethyl metabolite also has a terminal half-life of about 4 hours according to current labeling.
No. Half-life refers to proportional decline in plasma concentration during the relevant elimination phase, not directly to the fraction of the original oral dose remaining in the body.
In an idealized repeated-halving model, about 3.1% of the terminal-phase starting concentration remains after five half-lives. With an approximately 4-hour terminal half-life, that corresponds to about 20 hours, but it is not a precise statement about detectability or clinical effect.
No. Half-life describes late plasma concentration decline, while duration of effect is a pharmacodynamic or clinical concept. Current ED studies demonstrate response for up to about 4 hours, but this is not the same measurement as the approximately 4-hour terminal half-life.
Researchers estimate the terminal elimination-rate constant, λz, from the late approximately log-linear portion of serial plasma concentration data and calculate terminal half-life as ln(2) divided by λz.