Entecavir Side Effects: What to Expect and When to Call Your Doctor

Entecavir (Baraclude) is generally well tolerated—the majority of patients taking it experience few or no side effects. In clinical trials, most people taking the drug reported only mild, manageable symptoms. However, entecavir does carry three important boxed warnings, and understanding them can help you recognize when to seek urgent care.

Common Side Effects of Entecavir

In clinical trials comparing entecavir to placebo in patients with chronic hepatitis B and compensated liver disease, the most frequently reported side effects were mild:

• Headache — the most commonly reported symptom
• Fatigue
• Dizziness
• Nausea
• Elevated blood glucose (glycosuria, noted on lab tests)
• Elevated liver enzymes (ALT) — detected on lab work, usually transient

These symptoms are usually mild and often improve as your body adjusts to the medication. If any of these side effects are persistent or bothersome, discuss them with your prescriber—but do not stop taking entecavir without medical guidance.

Boxed Warning #1: Severe Acute Hepatitis B Exacerbations Upon Stopping

This is the most important warning for all patients on entecavir: do not stop taking it without talking to your doctor. Stopping entecavir abruptly can cause a sudden and severe worsening of hepatitis B—called an acute exacerbation. This is characterized by rapid spikes in HBV DNA levels and liver enzyme levels, which can sometimes lead to liver failure.

If you stop entecavir for any reason—even temporarily due to a pharmacy stockout—your doctor must monitor your liver function (ALT, bilirubin) closely for at least several months.

Boxed Warning #2: Lactic Acidosis and Hepatomegaly with Steatosis

Like other nucleoside analogue drugs, entecavir has been associated with a rare but potentially fatal condition called lactic acidosis (a dangerous build-up of lactic acid in the blood), sometimes accompanied by severe liver enlargement and fat in the liver (hepatomegaly with steatosis).

Risk factors for lactic acidosis include: being female, obesity, prolonged nucleoside analogue use, and existing liver dysfunction.

Call your doctor immediately or go to the emergency room if you experience:

• Unusual muscle pain, especially with weakness
• Trouble breathing or rapid/shallow breathing
• Stomach pain or nausea/vomiting that is persistent
• Feeling cold, dizzy, or very weak and tired
• Swelling in the belly or yellow skin/eyes (jaundice)

Boxed Warning #3: HIV Resistance in Untreated HIV/HBV Co-Infection

Entecavir has weak anti-HIV activity. If you have undiagnosed or untreated HIV and take entecavir alone, the virus may develop resistance to certain HIV medications, making future HIV treatment harder. Before starting entecavir, your doctor should test you for HIV. If you are HIV-positive, you must also be receiving fully suppressive antiretroviral therapy (HAART).

Other Side Effects to Know About

Less common side effects reported in post-marketing experience include:

• Decreased kidney function — particularly in patients with pre-existing kidney disease or those taking other drugs that affect kidney clearance
• Alopecia (hair loss) — rare, reported in post-marketing surveillance
• Anaphylactoid reaction — rare, but seek emergency care immediately if you have hives, difficulty breathing, or swelling of the face/throat

Monitoring While on Entecavir

Your doctor will typically monitor the following while you are on entecavir:

• Liver function tests (ALT, AST, bilirubin) — typically every 3–6 months
• HBV DNA levels (viral load) — to confirm viral suppression
• Kidney function (serum creatinine) — especially if you have other risk factors for renal impairment

For more on medications that interact with entecavir, see our guide on entecavir drug interactions. If you're having trouble finding entecavir at your pharmacy, medfinder can help locate it near you.