It is important that you use no aspirin compounds since these completely block the effects of guaifenesin. The greatest source of patient error lies in the use of aspirin-related compounds, salicylate or salicylic acid, which interfere with guaifenesin at the kidney level. Skin readily absorbs these compounds. Many pain medications contain aspirin or have the word 'salicylate' or 'salicylic acid' as part of the contents. You cannot use these.
ALL plants manufacture salicylates, and many do so in large quantities.
Herbal medications, cosmetics and skin creams or lotions containing herbs are also high in salicylates and must be avoided. Among others, these include aloe, camphor, castor oil, witch hazel, ginseng and plant-derived vitamins. You must read all ingredients contained in anything you apply to your skin. These warnings do not apply to foods, including herbs and spices used in cooking even though some contain salicylates. Apparently, the amount is insufficient for blocking because of the action of the digestive tract and because the liver processes small amounts by a process called glycination.