Diphenhydramine
Mechanism of action:
Histamine is secreted within the nose from mast cells and basophils, and plays an important role in an allergic reaction. It increases the vascular permeability and causes the nasal passages to become engorged, hence affecting the vascular tissues of the nasal epithelium. This results in sneezing, nasal congestion and rhinorrhea (Nacleiro 1988). Post nasal drip into the throat can be the primary cause of cough irriatation in the common cold. (Curley et al.1988) H1-Antagonists block these reactions, thereby eliminating the symptoms of cough caused by post nasal drip (Nacleiro 1988, Welliver 1990)Contra-indications:
There has been no evidence shown that antihistamine are contra-indicated in any disease state. Caution should be used when patients take diphenhydramine and other sedating medications, including benzodiapepines. (Micromedex, 2004) It is also advisable to avoid first generation anti-histamines in patients with a productive cough (Micromedex, 2004)Disease state information:
H1-Antagonists are used primarily to reduce the symptoms arising from an inflammatory response to mast cells and basophils. All evidence suggests that anti-histamines reduce the sneezing, nasal congestion, rhinorrhea and cough associated with the inflammatory response.Protocols for the use
Packman et al. (1991) performed a controlled crossover study in 20 healthy volunteers. They utilized an aerosol-induced cough response to reflect the effectiveness of diphenhydramine as an anti-tussive agent. Initially patients received 25mg/mL of either diphenhydramine or placebo. The active group showed the effectiveness of diphenhydramine at 15 minutes which continued over the entire 4-hour test period. In the placebo group, however, cough counts did not significantly change from baseline. Packman et al concluded that 25mg of diphenhydramine is effective in suppressing cough for 4hrs. The second study was performed by Lilienfield et al. (1976) which was a double-blind crossover study. They evaluated the anti-tussive effectiveness of diphenhydramine in chronic coughs related to bronchitis. The dose given to the active treatment group was either 25 or 50 mg every 4hrs for four consecutive doses. The result showed that there was a significant decrease in frequency of coughs in both the 25mg and 50mg doses compared to the placebo group. The active group, especially the 50mg group, did report a higher frequency of side effects including drowsiness. Lilienfield et al concluded that doses of either 25mg or 50mg suppressed coughs in patients with chronic bronchitis. Overall Diphenhydramine has shown to be effective as an anti-tussive agent in suppressing coughs.
