A cold sore consists of a group of small, painful blisters that appear around the mouth and lips, and in some cases, the nose, cheeks and inside the mouth. Cold sores are caused by the herpes simplex virus (HSV). They’re also known as herpes simplex labialis or fever blisters.

The symptoms of a cold sore are usually most severe the first time you get one. Once you experience the first outbreak, your body will create antibodies. Some people will never have another infection, but others do.

Are cold sores herpes?

A cold sore is a form of herpes. The herpes simplex virus type 1 (HSV-1) causes cold sores, whereas the herpes simplex virus type 2 (HSV-2) causes genital herpes. Even though HSV-1 commonly causes cold sores around the mouth and lips and HSV-2 causes sores around the genitals, both viruses can cause sores in both areas.

How common are cold sores?

Cold sores are extremely common. Up to 90% of the population have experienced HSV1. However, it’s important to note that cold sores aren’t usually a serious medical condition and there are many things you can do to treat your symptoms.

Cold sores typically appear around the mouth and lips. After being infected, it may take up to 20 days for the sores to develop, usually emerging near the area where the virus entered the body. Along with the sore, some people may experience swollen gums, swollen neck glands, muscle aches, or a fever.

What does a cold sore look like?
Cold sore blisters often start as small, fluid-filled bumps surrounded by red, swollen skin. After bursting, the cold sore dries up and forms a scab.

Stages of a cold sore
Cold sores develop in stages, meaning you won't suddenly get an open sore. Preventative medication can help reduce symptoms before they fully appear. The stages include:

1. Tingling, itching, or burning sensation.
2. Red, swollen, and painful area as blisters form, 6-48 hours later.
3. Blisters burst, releasing fluid, lasting 2-3 days.
4. A scab forms, which may crack or bleed.
5. The scab falls off, and the blister heals within 10-14 days in most cases.

How long does a cold sore last?
Cold sores typically clear up in around 10 days, though the duration varies. Triggers and the severity of the outbreak can influence recovery time. Fortunately, treatments can help speed up healing and ease discomfort.

The side effects of taking a commonly used cold sore tablet (aciclovir) will vary between individuals. The following side effects are common and occur in more than 1 in 10 to 1 in 100 people who take aciclovir: 

• Headaches. 
• Dizziness. 
• Nausea or vomiting. 
• Abdominal pain. 
• Diarrhoea. 
• Itching or an itchy rash. 
• Skin is sensitive to sunlight. 
• Tiredness.
• Fever.

If you experience any of the above side effects and they’re bothering you or not going away, then you should talk to your doctor or pharmacist. You don’t need to stop taking the medicine unless any of the symptoms are severe or you start getting symptoms associated with an allergic reaction like trouble breathing or swelling of the face or lips. If this does happen, you should stop using the medication immediately and call 999. 

If you have a fever it’s always important to contact a doctor to check for any other underlying causes for this. It’s important to read the information leaflet to check a full list of potential side effects, interactions and contraindications before taking any medication. 

Cold sore tablets and other medications are usually effective but they may not work for everyone. If you’re unsure about whether this treatment is right for you, then you should contact your doctor or pharmacist to ensure you’re making the right decision for your health. If you’re taking other medication, then a medical professional can tell you if it’s safe to take both medicines together.

List of contraindications

If you’re under 18, pregnant or breastfeeding, elderly, suffer with kidney problems or have a suppressed immune system it’s important to talk to your doctor before you start this treatment. You shouldn’t take cold sore tablets if you’re allergic to active ingredients in the medicine or similar medications.

Medications that interact with cold sore tablets

You must tell your doctor if you’re taking any of the following medications before taking cold sore tablets like aciclovir:

• Aminophylline or theophylline for asthma.
• Cimetidine for stomach ulcers.
• Mycophenolate mofetil after an organ transplant.
• Probenecid for gout.
• Certain medications used in the treatment or prevention of HIV

• Aciclovir (including Zovirax) (2019) NHS [accessed 03 November 2021]
• Cold Sore (2021) NHSinform [accessed 03 November 2021]
• Cold Sores (2020) NHS [accessed 01 November 2021]
• Scenario: Herpes Labialis (Cold Sores) and Gingivostomatitis (2021) NICE [accessed 26 November 2021]
• Aciclovir Tablets BP 400mg (2020) EMC [accessed 29 November 2021]