Pharmacology · Hepatitis B antivirals
Entecavir !
An oral first-line antiviral for chronic hepatitis B. It strongly suppresses the virus; stopping it can cause a severe hepatitis flare.
Updated 2026-11-01
The figure shows entecavir triphosphate in HIV-1 reverse transcriptase (Q151M variant) bound to DNA, a related enzyme; no HBV polymerase structure with entecavir is available.
Mechanism of action
Entecavir is a guanosine analogue. Cellular kinases convert it to entecavir triphosphate, which competes with dGTP and blocks all three activities of the HBV polymerase: base priming, reverse transcription of the negative DNA strand from pregenomic RNA, and synthesis of the positive DNA strand. Viral DNA in the blood falls sharply, but the cccDNA in liver cell nuclei remains.
- Cellular kinases make entecavir triphosphate
- Competes with dGTP at the HBV polymerase
- Blocks priming, reverse transcription and positive-strand synthesis
- HBV DNA falls; cccDNA in the nucleus remains
Pharmacokinetics
- Absorption
- Oral, on an empty stomach (food reduces absorption)
- Metabolism
- Not a substrate, inhibitor or inducer of CYP450 enzymes
- Elimination
- Mostly unchanged in the urine (filtration and tubular secretion)
- Half-life
- Terminal 128 to 149 hours; effective accumulation half-life about 24 hours
Indications
- Chronic hepatitis B with active viral replication and raised transaminases or active liver disease, in adults and children from 2 years Viral hepatitis →
Adverse effects
Common
- Headache
- Fatigue
- Dizziness
- Nausea
Contraindications
- None listed on the label apart from hypersensitivity
Key interactions
- Drugs that reduce kidney function or compete for tubular secretion
- May raise entecavir levels or those of the other drug
- Lamivudine-resistant HBV (rtM204I/V with or without rtL180M)
- Lower susceptibility to entecavir; resistance appears more easily
What is monitored
- HBV DNA and liver tests during treatment
- Liver tests for at least several months after stopping
- HIV test before starting
- Kidney function, especially in older people and transplant recipients
Administration and safety
- Oral, on an empty stomach: at least 2 hours after a meal and 2 hours before the next
- Tablets and oral solution are interchangeable
Nursing care
- Teach not to stop the medicine without medical advice, because hepatitis can flare
- Teach the empty-stomach timing
- Teach to report jaundice, dark urine, abdominal pain, unusual tiredness or fast breathing
- Remind that it does not prevent transmission; household contacts and sexual partners are vaccinated
Pregnancy and breastfeeding
Human data are insufficient to judge the risk; animal studies at usual exposures showed no harm.
Test yourself
Think of the answer, then tap the question to see it.
Which three HBV polymerase activities does entecavir block?
Priming, reverse transcription of the negative strand and synthesis of the positive DNA strand.
How is entecavir taken in relation to meals?
On an empty stomach, at least 2 hours after and 2 hours before a meal.
Why is HIV status checked before starting entecavir?
In someone with HIV who is not on full antiretroviral therapy, it can select HIV resistance.
What may happen when entecavir is stopped?
A severe acute hepatitis flare; liver tests are followed for months.
Related drugs
Related conditions
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - ENTECAVIR tablet (DailyMed (U.S. National Library of Medicine), 2026); Hepatitis B (World Health Organization, 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025).
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Representative model, for educational purposes only.