A cold or flu infection occurs when airborne viruses enter the nasal tract and trigger a reaction of the body’s immune system. NasalGuard Gel is totally drug-free and therefore will not cause any side effects (unlike some tablets which cause headaches, dry mouth, dizziness, drowsiness, etc.)If you would like to reduce the likelihood of getting sick from cold and flu viruses then use NasalGuard gel before symptoms begin and throughout the cold and flu season. http://www.nasalguard.co.uk/Cold_&_Flu_Remedies_&_Treatments_a/279.htm