Allergies and asthma are indications that your body has increased its production of histamine a neurotransmitter that regulates water metabolism. This is why antihistamines, drugs which block this effect, are used to dry up the watery eyes and runny noses of allergies and colds.
However, in his book Your Body's Many Cries for Water, Dr F Batmanghelidj points out that antihistamines are potentially dangerous because they interfere with the body's attempt to regulate its water supply. He believes that a consistent programme of six to eight glasses of water and half a teaspoon of salt (a natural antihistamine) daily, will normalise the body's water and histamine levels (Townsend Letters, Feb/Mar 1997).
Apart from interfering with the body's attempts to correct dehydration (by releasing histamine), other side effects of antihistamines include dry mouth, drowsiness, blurred vision, diarrhoea, nausea, vomiting and difficulties in passing water.