I have had a struggle off and on for years with this malady. I do believe in most cases it is the result of an infection- viral or fungal. The things that help me most are antiinfective herbals. I usually take three capsules of goldenseal and olive leaf TWICE a day for several days. If it is severe, I add six - ten drops of grapefruit SEED extract in water ( chased with juice as it tastes yucky ). These all target a broad variety of infections and usually make a noticeable difference. I also take aleve or ibuprofen, but the herbs seem to GET rid of the problem a lot faster.
Excellent herbal antiinfectives : andrographis, grapefruit seed extract, cat's claw, and olive leaf extract . If you have an infection and want to avoid antibiotics, these are scientifically legitimate and have been studied. I usually have good results with three capsules TWICE daily ( six total ) for several days. For colds, flu, viruses, etc.. These are safe and QUITE helpful!!
My doc has said he thinks costo is usually a viral infection, and these herbs are effective for many bacteria, viruses and fungii. More info on these can be found at Sloan Ketttering Memorial website or Pubmed.