When your doctor prescribes acyclovir for cold sores, shingles, or other viral infections, you might wonder: "Will this make me feel worse before I feel better?" Like any medication, acyclovir can cause side effects, but most are mild and temporary. Let's break down what you might experience – and when you should call your doctor.

The Common (Annoying But Harmless) Side Effects

About 1 in 10 people notice these minor reactions:

• Headache – Like a dull pressure that comes and goes


• Nausea – That "I shouldn't have eaten that" feeling


• Dizziness – Especially when standing up quickly


• Diarrhea – Usually lasts just a day or two

These typically start within the first few days and fade as your body adjusts. Pro tip: Taking acyclovir with food can help prevent nausea.

The Rare But Serious Reactions

While uncommon (affecting fewer than 1 in 1,000 people), these require immediate medical attention:

• Allergic reactions – Swelling (especially of the face/lips), difficulty breathing, or hives


• Severe dizziness/fainting – Like the room is spinning uncontrollably


• Unusual bruising/bleeding – Nosebleeds or gums bleeding without cause


• Dark urine or yellow skin – Signs of liver stress

Real People, Real Experiences

Sarah, 28, shares: "I had a mild headache after my first dose, but drinking extra water helped. By day 3, I forgot I was even taking medication."

Meanwhile, James, 45, notes: "I got dizzy once when I took it on an empty stomach. Now I eat a banana first – zero issues."

When to Call Your Doctor

Reach out if:

• Side effects last more than 5 days


• You develop new symptoms after starting acyclovir


• You can't keep fluids down due to nausea

Remember: Most people tolerate acyclovir well. As with any medication, the benefits (stopping viral growth) usually outweigh the risks of temporary side effects.

For detailed dosage information, see acyclovir side effects and dosing guidelines.

The Bottom Line

Think of acyclovir like a bouncer at a club – it's there to kick out unwanted viruses, but might occasionally bump into you by accident. Stay hydrated, listen to your body, and don't hesitate to ask your healthcare team questions!