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

Lentocilin (benzathine benzylpenicillin, or penicillin G benzathine) is generally well tolerated, but like all medications — especially injectable antibiotics — it can cause side effects ranging from mild to serious. This guide explains what to expect after receiving a Lentocilin injection, which side effects are normal, and which require immediate medical attention.

Common Side Effects (Usually Not Serious)

These side effects are relatively common and usually resolve on their own within a few days:

Injection Site Reactions

The most common side effects are at the injection site. Lentocilin is administered as a thick suspension deep into the muscle, and it's normal to experience:

Pain or soreness at the injection site (can last 1-3 days)

Swelling, redness, or warmth at the injection site

A lump or firmness under the skin (induration) that may take days to weeks to resolve

Applying a warm compress to the site after the injection can help with pain and induration. Over-the-counter pain relievers (ibuprofen or acetaminophen) are appropriate unless your provider says otherwise.

Skin Reactions

Mild skin rash or hives (urticaria)

Itching (pruritus)

Contact your doctor if rash is widespread, severe, or accompanied by other symptoms like fever or difficulty breathing.

Jarisch-Herxheimer Reaction (When Treating Syphilis)

If you're being treated for syphilis, you may experience the Jarisch-Herxheimer reaction within 2-8 hours of your first Lentocilin injection. This is NOT an allergic reaction — it is your body's response to the dying bacteria. Symptoms include:

Fever (often 38–40°C / 100–104°F)

Chills and sweating

Headache and muscle aches

Worsening of syphilis skin lesions (temporary)

The Jarisch-Herxheimer reaction typically resolves within 12-24 hours. Manage with fluids, rest, and acetaminophen or NSAIDs. Tell your provider if you're pregnant before your injection, as this reaction can trigger contractions — your provider will want to monitor you.

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Serious Side Effects — Call Your Doctor or Seek Emergency Care

Seek immediate medical attention if you experience any of the following:

Anaphylaxis (Severe Allergic Reaction) — Call 911

Anaphylaxis occurs in approximately 0.05% of patients receiving penicillin and can be fatal if untreated. Symptoms typically occur within minutes to an hour after injection and include:

Difficulty breathing or wheezing

Throat tightening or swelling (laryngeal edema)

Rapid heartbeat or drop in blood pressure

Swelling of face, lips, tongue, or throat (angioedema)

Loss of consciousness or dizziness

This is why Lentocilin must only be administered in a clinical setting with emergency equipment (epinephrine, oxygen, resuscitation equipment) immediately available. The overall rate of allergic reactions to penicillin is 1-10%; severe anaphylaxis occurs in approximately 1 in 2,000 treatments.

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Special Warning: Soy Allergy (Lentocilin-Specific)

Unlike Bicillin L-A (the U.S. equivalent), Lentocilin contains soy phospholipids. Patients with a known soy allergy may experience hypersensitivity reactions, including anaphylactic shock. Always tell your healthcare provider about soy or food allergies before receiving Lentocilin.

Rare But Serious Side Effects to Know

C. difficile-associated diarrhea: All antibiotics can disrupt gut bacteria. If you develop severe, watery, or bloody diarrhea during or within 2 months of treatment, contact your provider immediately. This can range from mild diarrhea to life-threatening colitis.

Sciatic nerve injury: Injection in the wrong site (not deep into the upper outer quadrant of the buttock) can injure the sciatic nerve. Report any sudden leg pain, numbness, or weakness after injection immediately.

Hemolytic anemia: Rare immune-mediated destruction of red blood cells. Symptoms include unusual fatigue, pale skin, or jaundice.

Neurotoxicity / seizures: Very rare; more likely with high doses or inadvertent IV administration. Any seizure after penicillin administration requires emergency evaluation.

What to Tell Your Provider Before Your Injection

Always tell your provider before receiving Lentocilin if you:

Have ever had an allergic reaction to penicillin, amoxicillin, or cephalosporins

Have a soy allergy (Lentocilin contains soy phospholipids)

Are allergic to lidocaine or other local anesthetics (Lentocilin's diluent contains lidocaine)

Are pregnant (important for monitoring Jarisch-Herxheimer reaction)

Have heart, liver, or kidney disease

Take methotrexate, oral contraceptives, blood thinners, or any other medications

Further Reading

Lentocilin Drug Interactions: What to Avoid and What to Tell Your Doctor

What Is Lentocilin? Uses, Dosage, and What You Need to Know in 2026