Final answer:
The genus Ephedra from the Gnetales is used for alkaloids like ephedrine and pseudoephedrine, which are effective in treating nasal congestion. These compounds are found in prescription and some over-the-counter decongestants but are regulated due to their similarity to amphetamines.
Step-by-step explanation:
The genus from the Gnetales that provides alkaloids used for nasal congestion is Ephedra. Ephedra is known for its production of the compounds ephedrine and pseudoephedrine. These alkaloids are beneficial in treating nasal and bronchial congestion because they can constrict blood vessels, reducing swelling and the production of mucus. Although these compounds are effective, they are similar to amphetamines in chemical structure and effects, which has led to restrictions in their availability, often requiring a prescription for use in cold medications. Pseudoephedrine, a stereoisomer of ephedrine, is particularly noted for its use in over-the-counter decongestant medications to relieve nasal congestion. It is important to note that due to the potential for misuse in the illegal production of methamphetamine, there are controls on the sale and distribution of products containing these alkaloids.