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

Firvanq (vancomycin hydrochloride for oral solution) is an antibiotic used to treat C. diff and staphylococcal enterocolitis. Like all medications, it can cause side effects — though many patients tolerate a 10-day oral course without significant problems. Here's what you need to know: what side effects are common, which are serious, and exactly when to call your doctor or seek emergency care.

How Is Firvanq Different from IV Vancomycin for Side Effects?

This is an important distinction. IV (intravenous) vancomycin is associated with a much wider range of systemic side effects because it enters the bloodstream directly. Oral Firvanq, by contrast, has very poor systemic absorption — less than 10% of the dose is absorbed into the bloodstream in most patients. This means oral vancomycin primarily acts locally in the gut, and many of the classic vancomycin side effects (like Red Man Syndrome from IV infusion) are not typically seen with the oral form.

That said, some patients — particularly those with severe inflammation of the intestinal lining (colitis) or kidney problems — can absorb more vancomycin than expected, increasing the risk of systemic side effects.

Common Side Effects of Firvanq

These side effects are the most frequently reported and are generally not dangerous. Contact your doctor if they persist or worsen:

• Abdominal pain or cramping — can occur as the antibiotic acts on gut bacteria; this may overlap with or be difficult to distinguish from C. diff symptoms themselves
• Nausea — taking Firvanq with food may help reduce nausea; the grape-flavored solution can also be mixed with a small amount of water
• Vomiting — less common but reported; persistent vomiting that prevents you from completing your dose should be reported to your doctor

Serious Side Effects of Firvanq — Call Your Doctor

These side effects are less common but require prompt medical attention:

• Nephrotoxicity (kidney damage) — kidney problems have occurred following oral vancomycin therapy, and can happen during or after treatment. Risk is highest in patients over 65, those with pre-existing kidney conditions, and patients taking other nephrotoxic drugs (like aminoglycoside antibiotics or NSAIDs). Watch for: decreased urination, swelling in ankles/feet, unusual fatigue, or increased thirst.
• Ototoxicity (hearing damage) — hearing loss or tinnitus (ringing in the ears) has occurred in patients receiving vancomycin. This risk is higher in patients who absorb more vancomycin systemically (those with colitis or renal insufficiency). Report any new ringing in the ears or hearing changes to your doctor immediately.
• Increased or varying vancomycin blood levels — patients with colitis or renal insufficiency may absorb unusually high amounts. Symptoms of high vancomycin levels include: stomach pain, nausea, cramps, and slow or irregular heartbeat. Seek immediate care if you experience these.
• Superinfection — use of any antibiotic can promote the overgrowth of bacteria that are resistant to that antibiotic. If you develop new or worsening diarrhea, fever, or signs of infection while on Firvanq, contact your provider — you may have developed a superinfection.
• Allergic reaction — Firvanq is contraindicated in patients with known hypersensitivity to vancomycin. Signs of a serious allergic reaction include hives, swelling of the face/lips/throat, or difficulty breathing. Seek emergency care immediately if these occur.

Who Is at Highest Risk for Serious Side Effects?

Your risk of serious side effects from Firvanq is higher if you:

• Are 65 years of age or older
• Have pre-existing kidney disease or reduced kidney function
• Have severe colitis (inflammation throughout the intestinal lining), which increases absorption of vancomycin into the bloodstream
• Are also taking aminoglycoside antibiotics (gentamicin, tobramycin, amikacin) or other nephrotoxic drugs — this combination increases kidney damage risk

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When to Call Your Doctor vs. Go to the ER

Call your doctor if you experience: persistent nausea or vomiting, mild abdominal pain, ringing in the ears, reduced urination, or new symptoms while on treatment.

Go to the ER immediately if you experience: difficulty breathing, severe swelling of the face or throat, irregular heartbeat, sudden hearing loss, or severe stomach pain with no urine output.

For information on medications and foods that can interact with Firvanq, see our companion article on Firvanq drug interactions: what to avoid and what to tell your doctor.