There are two proposed mechanisms by which menthol and the other herbal compounds in Vicks VapoRub operate to relieve coughing. As with garlic, one method is absorption via the skin into the bloodstream. The other is direct irritation of the mucous membranes in the throat, causing them to shut up shop. Of these two methods, experts believe that the most effective way for Vicks VapoRub to work is through skin absorption. As far as its effectiveness, studies have shown that Vicks VapoRub is more than 90% less likely to cause respiratory problems when used as directed. It may also be helpful in reducing symptoms such as fever, sore throat, and muscle aches caused by colds and flus. However, because it can be absorbed through the skin, individuals who suffer from allergies or asthma should use caution if they plan on using this product regularly. Vicks VapoRub is not recommended for children under six months old due to the potential risk of sensitization to methyl salicylate. For the same reason, people who are already sensitive to aspirin should not use Vicks VapoRub because it contains a large amount of aspirin. Finally, those who have severe lung problems such as emphysema should not use Vicks VapoRub because even small amounts of alcohol or tobacco smoke can damage lungs that aren't fully healed from previous injuries or illnesses.