I have been suffering from the same lip/facial swelling for the past year - it just started happening out of nowhere. I have been through alot of distress and visited MANY doctors in order to find out what was wrong with me, and the information that I have gathered may assist you.
The condition is known as Idiopathic Angioedema or Urticaria (look it up through the Google Directory on the web [www.***********] - there is loads of helpful information about the condition). It is a result of an allergy to something, but it can be very hard to pinpoint what is actually causing it. I suspect mine may be a food allergy, but I also suffer from hayfever and have experienced the swelling just from being itchy and sneezing due to dust. The swelling itself is due to an over-production of histamine in the body as a result of something you're allergic to.
To prevent any outbreaks (as I know how much it can effect your everyday life), I take an anti-histamine EVERY NIGHT before I go to bed (as prescribed by my doctor). Since I have been doing this, I have had no outbreaks. However, it is important that you try to get to the bottom of what is causing the swelling. I have been referred to a Specialist to have an allergy test in order to find out a likely cause, as I don't want to have to take an anti-histamine tablet every day for the rest of my life!
Start writing down foods and things that you have come in contact with leading up to the swelling, this will assist a specialist in helping you to find a cause.
Does your doctor also prescribe you steroid tablets and a steroid cream for when you do have an outbreak? This helps the swelling to go away faster.
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[This message has been edited by kitaki (edited 05-08-2003).]