The pharmacokinetics of Desogestrel explain why the pill works consistently, why it must be taken daily, and how it maintains strong ovulation suppression. Because Desogestrel is a prodrug, understanding its absorption and conversion into etonogestrel is crucial for appreciating its effectiveness as a modern progestin-only contraceptive.

This article covers its ADME profile:

Absorption
Distribution
Metabolism
Excretion

Absorption of Desogestrel

Rapid & Efficient Absorption

After oral administration of a 75 mcg tablet:

Desogestrel is quickly absorbed from the gastrointestinal tract
Absorption is not significantly affected by food

Bioavailability

Because Desogestrel is a prodrug converted to etonogestrel:

Its absolute bioavailability cannot be measured directly
Etonogestrel bioavailability is estimated at approximately 70%

Peak Plasma Concentrations

Peak etonogestrel levels appear around 1.5 hours after ingestion
Stable serum concentrations are achieved after 4–5 days of daily use

Clinical Impact

This rapid absorption:

Allows effective mucus thickening within 48 hours
Begins ovulation suppression within several days
Enables 24-hour coverage with once-daily dosing

Distribution of Desogestrel / Etonogestrel

Once converted into etonogestrel, the active hormone distributes throughout reproductive tissues.

Protein Binding

Etonogestrel is 95–98% protein-bound
Primarily binds to:
- Albumin
- Sex hormone-binding globulin (SHBG)

Volume of Distribution

Moderate distribution across tissues
High affinity for progesterone receptors in:
- Cervix
- Endometrium
- Hypothalamus
- Ovaries

Clinical Meaning

High protein binding ensures:

Stable serum hormone levels
Reduced hormonal fluctuations
Reliable contraceptive effects

Metabolism of Desogestrel

Prodrug Conversion

Desogestrel must first be activated in the liver:

Desogestrel
↓ (hepatic oxidative metabolism)
3-keto-desogestrel (etonogestrel – active metabolite)

Primary Enzymes Involved

Cytochrome P450 system
Mainly CYP3A4

Metabolic Pathways

Oxidation into etonogestrel
Hydroxylation
Conjugation (glucuronidation and sulfation)

Drug Interactions

Because CYP3A4 plays a major role, enzyme-inducing medications can reduce the effectiveness of Desogestrel, including:

Carbamazepine
Phenytoin
Rifampicin
St John’s Wort

These increase metabolism, decrease etonogestrel levels, and increase the risk of ovulation.

Half-Life of Desogestrel & Etonogestrel

Desogestrel Half-Life

Short, due to rapid conversion
Not clinically relevant on its own

Etonogestrel Half-Life

Approximately 25–30 hours

Why Half-Life Matters

The long half-life provides:

24-hour contraceptive coverage
A 12-hour window for late pills
Slow decline in hormone levels with minor delays

This stability explains why Desogestrel is more forgiving than older POPs.

Excretion of Desogestrel / Etonogestrel

Routes of Excretion

After metabolism, conjugated metabolites are excreted via:

Urine (40–60%)
Faeces (40–60%)

Etonogestrel itself undergoes minimal renal excretion.

Excretion Timeline

Most metabolites are eliminated within several days
No accumulation occurs with daily use

Breastmilk Transfer

Small amounts of etonogestrel may pass into breast milk, but:

No adverse effects on infant growth or development have been observed
Considered safe during breastfeeding

Pharmacokinetic Advantages Over Older Progestins

Compared with norethisterone or levonorgestrel, Desogestrel has:

Higher metabolic activation efficiency
More stable serum hormone levels
Longer half-life
Stronger ovulation suppression
Fewer androgenic side effects

These advantages explain its consistently high contraceptive performance.

Factors That Affect Desogestrel Pharmacokinetics

Absorption Reduced By

Vomiting within 3 hours
Severe diarrhoea
Malabsorption disorders

Increased Metabolism Caused By

CYP3A4 inducers
Certain antiepileptic medications
Herbal supplements such as St John’s Wort

Weight, Age & Smoking

These factors have minimal impact on Desogestrel pharmacokinetics, making it suitable for women over 35, smokers, and individuals with higher BMI.

Summary of Pharmacokinetics

Absorption: Fast, reliable, ~1.5 hours to peak levels.

Distribution: Highly protein-bound (95–98%), stable hormonal profile.

Metabolism: Liver conversion via CYP3A4 into etonogestrel.

Half-Life: Long (25–30 hours), enabling consistent 24-hour effectiveness.

Excretion: Eliminated evenly through urine and faeces.

Clinical Summary

Desogestrel’s pharmacokinetics create a stable, high-reliability contraceptive effect driven by consistent conversion into etonogestrel.

It provides:

Strong ovulation suppression
Predictable hormone levels
A forgiving dosing window
High contraceptive reliability
Suitability for a wide range of women

This pharmacokinetic profile explains why Desogestrel is considered the modern standard for progestin-only contraception.