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These are drugs which mimic the effects of naturally produced adrenaline, the messenger of the sympathetic nervous system (see p144) which produces the ‘flight or fight’ reaction. Sympathomimetics have various effects, but one local effect is to make small blood vessels (capillaries) contract. Thus they have an opposing effect to histamine. This is exploited in some nasal sprays for hay-fever and perennial rhinitis. The drugs concerned are phenylephrine (Neophryn), oxymetazoline (Afrazine) and xylometazoline (Otrivine).
These sprays make the capillaries in the nose contract providing immediate relief from congestion, but if used for more than two weeks they can-have adverse effects. The blood vessels become ‘hooked’ on the drug so that when the spray is discontinued they react by expanding, causing congestion again. These sprays are for short-term use only.
Some sprays combine sympathomimetics with antihistamines (eg Hayphryn and Otrivine-Antistin). Others combine sympathomimetics with antihistamines and antibiotics (Vibrocil) or with corticosteroids and antibiotics (Dexa-Rhinaspray). Sprays containing antibiotics are only used where there are signs of infection as well as allergy.
Sympathomimetics such as pseudoephedrine and ephedrine are sometimes combined with antihistamines in medicines taken by mouth, such as Congesteeze (pseudoephedrine and azatamine), Haymine (ephedrine and chlorpheniramine) and Sudafed Plus (psudoephedrine and triprolidine) When taken by mouth, the sympathomimetic helps to overcome the main side-effect of the antihistamine, drowsiness.
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