Here's a wee article from the NHS which better illustrates my point:
http://www.nhsdirect.nhs.uk/articles/article.a spx?articleId=494&PrintPage=1
Quote:
"Antihistamines can help to ease the symptoms of a cold because one of the side effects of some antihistamines is that they can help dry up mucus. Examples of antihistamines used in cold remedies are chlorpheniramine, brompheniramine and triprolidine. Doctors don�t usually recommend taking an antihistamine alone to help ease cold symptoms. However, antihistamines are often used in cold medicines along with a painkiller or an oral decongestant.
Antihistamines may cause drowsiness, dry mouth, constipation, difficulty passing urine, or blurred vision. They may also interact with other drugs that cause drowsiness, such as alcohol and some antidepressants and could lead to problems for people with glaucoma or prostate problems. You should check with a pharmacist or your GP before taking cold remedies containing antihistamines"
So it does dry up the mucus in a cold but can have side effects so check with your GP first!
PS If you google "cold remedies" and "antihistamine" you'll find that most cold remedies contain antihistamines as well as decongestants, whose stimulating properties counteract the drowsiness of the antihistamine