Final answer:
Adrenergic drugs, specifically phenylephrine and pseudoephedrine, are used as decongestants for treating colds. They target the adrenergic receptors to help clear mucus in the respiratory tract, while cholinergic drugs are not typically used for this purpose.
Step-by-step explanation:
For treating colds as a decongestant, adrenergic drugs are commonly used. Phenylephrine, an α₁-adrenergic agonist, is one such drug that is often found in cold medications. It works by binding to adrenergic receptors in the bronchioles, causing them to dilate, which helps clear mucus from the respiratory tract. Another adrenergic compound is pseudoephedrine, which is also used as a decongestant, but its use has been regulated due to its potential in the illicit production of methamphetamine. Yet another adrenergic substance is epinephrine, which is released in response to stress and can alleviate symptoms of colds and asthma by contracting the capillaries in the mucous membranes. In contrast, cholinergic drugs like atropine act on the parasympathetic system and are typically not used as decongestants.