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What causes cold sores?

Cold sores are caused by a virus called herpes simplex. (Herpes simplex viruses also cause genital herpes.) Most adults worldwide have been infected with this virus, usually when we were children and were kissed by someone with a tiny cold sore that they were not aware of. When a child first gets the infection, there may be no symptoms at all, or there may be a fever and a sore mouth that clears up after 2–3 weeks. 
After the first infection, the virus often remains. We seem to have difficulty in getting rid of it completely. It travels away from the mouth using the nerves of the face as a pathway. When it reaches the clusters of cells at the end of the nerves (which are called ganglia), it stops. The herpes virus DNA then remains quietly in the ganglia. Fortunately, it does not damage the nerves or interfere with their function.
In some people (about 1 in 12), from time to time, the herpes virus travels back the way it came, along the nerve towards the mouth. This is called activation. But instead of going to the inside of the mouth, it takes a slightly different path and ends up in the skin of the lips or nearby. When it arrives in the skin, it causes a cold sore.
After the cold sore heals, the virus goes back up the nerve to the ganglion again, where it rests. But it can become activated again at any time, and travel down the nerves to the skin to cause another sore. The virus always travels up and down the same or nearby nerve pathways, which explains why cold sores always recur in roughly the same place.

Infection with herpes simplex virus causes sore mouth (or no symptoms)

Herpes virus travels up nerves to the nerve ganglion where it remains

At a later date, herpes virus may travel down the nerves to the skin, causing a cold sore

The cold sore heals, and herpes virus retreats to the ganglion again

At a later date, herpes virus may travel down to the skin again, causing another cold sore (recurrence)

What activates the virus to cause a cold sore? No one really understands why the virus in the ganglion suddenly wakes up from time to time, and decides to go to the skin and cause a cold sore. But, in some people, there are some definite triggers that make this happen, such as:
  • bright sunshine
  • wind
  • other damage to the skin (such as ‘facial resurfacing’ procedures)
  • emotional stress
  • physical stress, such as having another illness
  • major dentistry, such as having a tooth removed
  • menstruation.

Written by: Dr Margaret Stearn
Edited by: Dr Margaret Stearn
Last updated: Thursday, March 4th 2010


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Comments on this article

Posted by Travo on 10/01/2017 at 09:44

I was sleeping last Sunday at around 4 a.m and woke up around 10 and before I went to sleep I didn't have an itch or anything. I woke up with an itch on my lip and some little transparent bubbles on my bottom lip. I was told that I had "fuego" fuego is something that pops up on your lip when you're not hydrated and you're around really hot places. I did kiss someone that same week on Wednesday but after that I didn't have contact with anyone like that. Do I really have a cold sore or is it "fuego"?

Posted by Optional on 21/10/2015 at 02:51

S it normal for me to get a cold sore at 14?

Posted by Optional on 02/04/2015 at 05:44

Hi I'm 14 and I have a huge prob. I recently got an cold sore on my mouth , wich I tought was fine cuz I sometimes get cold sores. But now its getting more and spreading all over my mouth , chin and in & on my nose. This is really embarrassing! I also have a red rash on my vagina and my legs. I think this could be a herpes virus ? But I'm not sure. Can someone please tell me what they think about this or give me home remedies to make it go away faster. I heard it takes 2-3 weeks to heal. I'm very disgusted and I have a camp this weekend. I don't wanna know what the people is going to think of me !! #PlzHelp - thanks !x

Posted by Optional on 24/03/2015 at 07:10

L-Lysine works really at the first sign and few days after

Posted by Optional on 26/02/2015 at 06:06

I absolutely never comment on anything but ccold sores have plagued me my whole entire life. They have embarrassed me during the most important times of my life like Christmas dances and proms. I drank orange juice alot as a kid and quickly noticed that it was my number one enemy. I got a fever blister without fail Everytime I drank it. From my Years of research the acid in it is what caused it. Kiwis do the same thing. After all of my years of agony and turmoil with these things I finally have the answer. I don't know if it works for everyone but I talked to a doctor and got acyclovir. There are absolutely no side effects that I have noticed and it's worked wonders for me. I always keep a prescription with me and since I am a pro at detecting the tingling or soreness before the cold sore pops out I just take an acyclovir at the first sign and it has worked for me everytime. The key is to get to it quickly and if I am ever worried that I may still get one I will put ice on my lip and it kills the progression on the cold sore. Also if it's too late for u and u get the cold sore anyways take a regular tea bag, I use lipton. And let it soak it really hot water then apply it to your sore. I usually just do it as long as I can while watching a tv show and it will minimize the pain and will significanly reduce the healing time. If u are in a bind and can't get to any of these things toothpaste has also helped dry up the sore plus the mint in it will help relieve some of the pain. Also I forgot to mention that lysine is a great product to keep on u. I have lysine ointment that I apply if I even think I may be getting one and its never let me down. Apparently the virus hates that supplement and has been very useful in many studies. Ok I think I have shared enough. I pray that this post helps someone.

Posted by Optional on 22/01/2015 at 04:11

It's alarming that you don't mention DIET as something that can trigger cold sores. I notice that if my body is too acidic, that they occur. This is a result of not having enough good foods like vegetables and protein, and more sugar and processed foods. The virus thrives in this type of environment and people should be aware that a healthy diet and exercise, keeps your body at a healthy pH, and the virus at bay.

Posted by Liza on 18/01/2015 at 08:43

I always get a cold sore around the same time of the year for every other couple of years. Some very big some small but still noticeable. I use abbreva or if I can get a prescription for zovirax (acyclovir). I also clean it with alcohol to dry out the blisters, apply ice every couple of hours. But I'd rather a cure bc they are embarrassing. I don't leave my house until the blisters go away and then I cover the scab with lipstick/gloss in order to leave the house hoping no one notices.

Posted by Optional on 05/01/2015 at 01:47

I agree about the citrus. I have avoided oranges for years and not one cold sore. I started drinking water with lemons in it and, bam, one showed up immediately. No more citrus for me.

Posted by Optional on 02/01/2015 at 01:03

i am 16 and have had coldsores for along as i can remember this month i have woken up with a huge coldsore in the top left corner of my mouth, the hurt so much they make me want to cry, the question i am asking is there anything that can prevent coldsores from coming any thing to stop them from getting them as bad as i do, because now I'm at this age i feel embarrassed to go out the house with it on my lip as it in my opinion is disgusting and I'm scared people will think it is disgusting to, even though i cannot help the matter

Posted by Bread on 31/12/2014 at 05:04

I've always tried different methods to try and heal the cold sore as quickly as possible and after many years, I've found the ideal method for getting rid of them (at least for me). I can fully recover from a cold sore in 5 days. The instant you feel an tingle, apply cold sore cream and maintain a high level of hygiene by always washing hands after touching the area. Next, eat a lot of vitamin C (I use oranges) as soon as possible and keep doing it until its a scab. The key step is that after you eat the oranges, apply ice to the area until it's numb and then keep it there some more. When you've had enough, dry your mouth carefully and apply a generous amount of the cold sore cream. Repeat the icing until it's scabbed and then just apply cream.

Posted by Optional on 24/09/2014 at 05:11

Iv been struggling with cold sore out break every month for the past 5 years. Recently its becoming even more regular with 1 sore out break then healing followed immediately with a cluster of sores. They dont repeat in the same area and becoming the Bain of my life. i have taken all on shelf treatments, at present using an over counter product but still no luck. Away to try anti viral from the doctors and a contraception to see if hormones are involved in outbreaks. i am feeling like its no use and completely helpless.

Posted by CJ on 19/08/2014 at 05:50

My daughters new boyfriend suffers from cold sores often and I gave him a fabulous new product to try...called Intercept CS you administer it to the area the minute you feel the "tingling" sensation and it PREVENTS the cold sore from developing. He said it works amazing and after using it on three occasions (in 1 month) he has now gone three months with out a cold sore flare up. Worth trying!

Posted by Optional on 20/07/2014 at 02:34

I have recurring cold sores, always in the same corner of my mouth. I have a mustache which I bleach regularly, but find that this does NOT cause a cold sore to appear. However, if a hair remains dark after bleaching (in that particular corner of my mouth) I will sometimes pluck it out. I have just proven to myself that this WILL cause the cold sore to reappear. This may be something that other people should try to avoid. I will from now on!

Posted by Optional on 24/06/2014 at 04:36

I suffered with cold sores all my life until I took a zinc supplement, then they where gone....the only time they reoccur now is when I get sick or run down, soon as I feel them coming on I take an extra zinc/multi vitamin tablet and it keeps them under control, hope this helps someone...

Posted by nomvula on 03/06/2014 at 09:08

this thing is irritating igot it in 2011 since then it never went off at first ijust though it was asmall thing.. it so itchy and dry on the left corner on my mouth, when it's dry ipeel it off but it just remains there im so tired of it please help!!

Posted by Optional on 18/05/2014 at 02:36

Hey, I have loads of ulcers at the bottom of my mouth and i am finding it impossible to eat or drink! What will help? Thanks xx

Posted by 44 yr women on 03/05/2014 at 06:16

On my chin right below my bottom lip occasionally when I pluck a dark hair bamb! a cold sore. Only once in a while but sure annoying. This time I turned to the internet and tried some of suggested remedies. I am not a doctor but what to share what I thought worked amazing. After trying, Tea tree oil, silver, magnesium spray, and alcohol, I felt non really worked so I tried liquid bandage. It cc Ones in a small brown bottle at any pharmacy. I always keep this stuff on hand for it works great, especially cuts on fingers that won't keep a bandaid on. It was almost time for bed so I thought I'd give it a whirl. It always stings when putting on any cut and it did sting when I out it on the sore. Should only last for 5 mins or so. Amazing I woke up today with the cold sore almost all gone. Also, liquid bandage coats and covers the cold sore so I couldn't play with it or spread it. I'm very happy with these results and what to share my findings. I just can't stand a cold sore. I would be interested to know if it worked for anyone else.

Posted by Optional on 20/09/2013 at 02:04

I iced my cold sore on and off for 9 hrs. In between icing I popped it a little and kept putting peroxide on it, it's 2 days later and its almost gone. I hope this helps somebody!

Posted by kath on 02/05/2013 at 12:31

hi, my mum has a cold sore on the bottom middle of her lip and its going black is that normal? i feel sorry for her please help.

Posted by rach on 08/04/2013 at 09:40

i have never had a cold saw before until i went to the dentist to have a tooth removed. two days later masive cold saw now i get them all the time.

Posted by Jo on 23/03/2013 at 10:05

1st cold sore :(.... Why? Same man for 24 years?

Posted by Bec on 18/02/2013 at 10:02

I have suffered with cold sores since I was 28. They are getting worse as I get older. I've never been to the doctors I just got on with them. But last October 2012 I had one after the other 9 in 6 weeks, also my tongue had lots of ulcers which where very painful and my tongue ached. I went to see my GP he also noticed a small cluster of scabs on my chin he said you have got acute fatigue an the only cure is 6 months complete bed rest, then he laughed, and said unless your a celebrity or royalty I doubt you will get it. He then hit me with the words herpes I was stunned. He gave me a mouth wash which worked for the ulcers. The cold sores are fading but the scabes look awful and are getting worse HELP!

Posted by Optional on 20/12/2012 at 10:45

Thanks so much

Posted by Fight the herpes on 19/10/2012 at 01:34

I've tried lots of pharmacies medications however I've found the best thing to use on cold sores is B O N J E L A..!! It's a nice cool tingling feeling and apply it before you go to sleep. Peel off the thin dry layer that bonjella forms ands its like magic. Excellent with VERY QUICK RESULTS!

Posted by Optional on 16/05/2012 at 12:34

I never had one in all my life until a year ago, when I went threw a very stressful time and wasn't looking after myself very well I find as soon as I feel one coming drink plenty of fluids and found a fantastic cold sore medicine in boots, it does sting a bit when first applied but disappear's very quickly. It is a boots own brand and a small bottle with clear liquid inside (i don't have it to hand to give the name).

Posted by Optional on 09/10/2011 at 02:02

I've started getting cold sores after a vaca to Florida when I was 20. I got a nasty sunburn. A couple months after the vaca I got one and went to my Dr. scared out of my mind. When he said the word "herpes" I thought I was going to vomit. He quickly calmed me down and asked me a few questions. We finally came to the conclusion it was the result of that sunburn. He explained about 80% of all people have the virus lying dormant in their system and it takes one of the triggers mentioned above to bring it out. For me, its too much sun or harsh facial cleansers or treatments. He explained also that as I grew older the outbreaks would be less often and less severe which thankfully is the case. I might get one once every year or two and it barely makes a bump. Easily passes for a small blind pimple. But be careful!!! You CAN easily transfer it to the genital area by touching your cold sore and then inadvertantly touching your genital area. Keep your hands away from your coldsore and wash your hands frequently or use antibacterial gel. To the person that pops the way to shorten the life of the cold sore is to keep it dry and to NOT POP IT! Popping it keeps the area moist and prolongs the healing process. It only gets super ugly once it opens/pops so I wouldnt use more than a medicated lip balm (to keep the skin pliable and comforable but NOT moist. Or use a prescription gel your doctor would provide. Hope this is helpful.

Posted by Aidan on 13/07/2011 at 07:28

I have been suffering from cold sores for years and after just getting another one badly I can see how I got it so if you want some advice, dehydration and sunlight are a big factor and the faster you get some cream on it the better! i recommend zovirax and not doing what i did (being a pale skinned person after taking in a lot of sunlight and not drinking water whilst on holiday, I drank alchohol until I was sick and didnt drink any water) no wonder I got one!

Posted by Optional on 21/06/2011 at 10:14

Try this - Colloidal Silver. Pour onto a piece of cotton wool/bud and dab the affected area. Works wonders !

Posted by Optional on 23/04/2011 at 12:05


Posted by Optional on 23/03/2011 at 04:02

I've been suffering from cold sores since I was 7. Being almost 40 and still suffering from these nasty blisters... I'm done! There are a few triggers for me. Illness, dehydration, sun and stress. For me they occur in the exact same place every time, which is very common. One thing I do when I feel an outbreak coming in puncture the blister right away (with a disinfected pin) and put on Abreva. I find that it prevents the blisters from spreading and taking up my entire lip! Once that horrible swollen and tingly feeling is gone, it's on the mend. You just have to deal with yellow crusted scab... which really is the worst part.

Posted by I'm with 'Savlon'! on 21/03/2011 at 06:00

Our family has found the same! It works wonders!

Posted by Optional on 08/03/2011 at 11:44

hi and im josh i always get cold sore from the sun, chocolate and sometimes i get a tooth out. and i always get it in the bottem left of my lip its really anoying becuase i get it all the time and i cant go to work becuase of them

Posted by sunny pony on 21/02/2011 at 07:06

i havent had a cold sore in years but recently i kissed my boy friend and i dont think he had a cold sore but ive got mine 4 days after we kissed! why is this?

Posted by Anonymous on 15/10/2010 at 10:31

Hi, I get cold sores all the time! I went to see my doctor and I told him about my problem, when i get them I want to crawl under a rock, i feel that its all that anyone see's. I felt one coming out about 10 days ago! by the time i got to the doctor and got a precription it was already out! It is in the scabbing stage now. The doctor precribed Zovirax ointment and apo-acyclovir tablets. 5 tablets a day for 5 days! Ive taken all the tablets already and ive used the cream as well, and its still there! :( usually it works, it was just out before i could get to the pharmacy! I hate them...I wish there was some kind of surgery you could get so you would never ever get one again...any such thing? lol

Posted by Ally on 02/09/2010 at 06:53

ive got a cold sore and i got it a couple of months ago for 2 weeks and it went away now it has come back a wekk before school! i want to get rid of it before school, can savlon cure it?

Posted by Optional on 26/08/2010 at 12:48

traveling alone here and always wondering if I will be robbed or held-up stresses me and traveling to different climates makes me have them, I just get those creams but it only prevents them from getting bigger......

Posted by Sigrid on 27/05/2010 at 03:18

I seemed to have the outbreaks under control for a number of years now with almost being able to predict an outbreak however, just this year they cold sores seem to recur fairly quickly one after another. I know stress can be a huge trigger but I wonder if my age and coming into menopause might also contribute which I guess will not leave me with much prevention options?

Posted by Optional on 19/05/2010 at 10:30

how do you get rid of them completly or will they still come back?

Posted by Optional on 17/04/2010 at 06:53

Handy hint. If you do suffer with cold sores, avoid vitamin C! Everytime I go near a citrus fruit I get a massive coldsore! I told my friends who suffer and they've noticed too! We cant all be wrong or wired up the same way!

Posted by Optional on 26/03/2010 at 12:33

A great tip that works for me when ive got them is to put vasaline on it at night, then by the morning has normally cleared.

Posted by Optional on 05/03/2010 at 03:50

Avoid dehydration and you'll avoid most outbreaks.... I get them EVERY time I go on a Big drinking binge (As I get severely dehydrated). I have also heard that Valtrex is a wonder pill.

Posted by becky on 10/12/2009 at 01:23

Hi. I always seem to get cold sores in the corners of my mouth. As i am a naturally tired person, i like to yawn a lot. It really hurts when i yawn & it often starts's tearing me apart. Please HELP!!!

Posted by zoe on 14/11/2009 at 11:56

my father is 40 and he has only just started getting cold sores is that normal?

Posted by Anonymous on 23/07/2009 at 05:27

why do i get cold sores on my nose

Posted by Dean on 22/04/2009 at 04:21

I have suffered from Coldsores for as long as I can remember...Here's my top tips for keeping them at bay; Sleep well - Being tired really increases the chances of getting a coldsore. Drink plenty of water. In my experience being dehydrated is 'the' number one cause of coldsores...I have reduced my coldsore outbreak from 6/year to 1/year (and that 1 is usually because I've forgotten to drink well enough). Coldsores have a high chance to breakout when it's sunny...i.e. You dehydrate faster. Coldsores have a high chance to breakout when you have drunk alcohol. Two-fold problem here, you get both tired and dehydrated from the alcohol. Drinking plenty of water after a night out, will help with your headache and coldsore problem the next morning. Good luck, fellow sufferers. ;)

Posted by emma on 21/02/2009 at 12:11

i have suffered with coldsores ever since my bf had one about 3 years ago and he hasnt had one since and i am still getting them is that normall?

Posted by Anonymous on 04/02/2009 at 05:21

I have suffered from cold sores all my life... and quite severely! My father recently bought me a "Oral light" (also used for ulcers in the mouth). It is a instrument that looks much like a electric toothbrush but with a transparent end where the brush should be. Apply this end to where you feel the tingle for 1-2min every for often as you like. You feel a slight warm sensation. It works wonders for me!! Sometimes if you get it early enough you don't even blister! I use it in conjunction with creams. Wow. It's an amazing contraption. I love it and don't go any where without it. look at this link for what it looks like: This is not the one I used but looks similar.

Posted by amy on 04/02/2009 at 02:30

i have a coldsoe n my cheek, on the left hand side. its sore and is starting to look yellow is this normal?

Posted by er on 03/02/2009 at 08:05

i know i have some type of herpes, but i never went to the doctor. iI KNOW ITS BAD, BUT IT IS SO EMBARSSING....

Posted by SAVLON on 29/01/2009 at 08:59

Like many people I suffered the embarassment of large open sores.I have tried patches [ seemed to make matters worse by spreading to other parts of the mouth ] and various creams all seem to help but did not clear up the problem. My solution SAVLON I have found it rapidly clears and cleans the wound sometimes stopping it in its tracks and not developing any scab or sores, its purpose is to heal skin and I have found it works for me. I have not tried any similar products like germolene etc. so I can not comment on these products but as each persons skin will react differently its worth trying the various options. I can only restate my experience of almost immediate healing by using SAVLON. I started using it with a cream to heal the skin then using it instead of the creams.If you suffer badly then give it a try it might work for you. I hope this helps you it is not an endorsement of SAVLON merely an honest desire to share a solution that works for me. thanks AM

Posted by vicky p on 14/01/2009 at 11:36

can cold sores (herpes) be caught by having oral sex ?


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