Hi, I have just started aciclovir and now have developed mouth ulcers on my lips for the first time in 14 years. I'm just wondering if anyone has any ideas about why this might be. Can anti-virals re-stimulate latent viruses? (seems unlikely but who knows...) I developed ME in 2002 whilst travelling in Sri Lanka. I was 18, it was a stressful time, my immune system was probably under attack from various unknown foreign bugs, then I went and got an immunisation for Japanese Encephelitis as I was travelling on to North Thailand. It's only recently through reading back my old travel journals that I realised that this was probably the trigger as the illness has been progressive for me. In the days after the jab I developed the kind of tiredness that we all know so well, I'd been tired before but not like this. Then within about 3 days I'd developed a huge and very painful mouth ulcer, thought it would go away - just kept getting worse. It lasted about two weeks and made me very miserable until I managed to track down some topical cream and it went away. Now, 14 years later, I have moderate/severe ME (mostly housebound). My worst symptoms are muscle aches, neck and shoulders particularly, PEM, constant chest infections and sore throats, sleep disruption, I'm not affected so much by cognitive symptoms. I was given aciclovir 400mg per day by my doctor, as he said there was some evidence of viral reactivation (I hadn't actually told him much about the experiences in Asia and the mouth ulcer as I hadn't read back through my old diaries at this point). So... I took 400mg for two days and then y'day and today only 200mg as it was affecting my stomach. Today, I've got that tingly feeling and small blisters beginning to appear .... what is going on? Wouldn't it be killing the herpes virus? Why would it be giving me ulcers? I've not had an ulcer on my lips for since the first one at the onset of the illness. Having read a bit about the herpes virus today I do think it's probably been in my body all this time as I've had recurrent blood blisters inside my mouth and on my tongue that come if I eat acidic or sharp foods. I didn't class them as mouth ulcers as they look different but I've read today that herpes virus can cause these blood blisters. So taking aciclovir would seem to be the thing to do... but I want to know why it would be giving me more symptoms? Is this a good sign, should I keep taking it? I'd love to hear from anyone whose had experiences with aciclovir or anyone who has a better understanding of viruses and anti-virals than I do. Thanks in advance.