Dosages for phenylephrine are 2 to 5 milligrams subcutaneously every one to two hours as needed for hypotension and 2 to 5 milligrams subcutaneously every one to two hours as needed for shock, according to Drugs.com. Other dosages include 10 to 20 milligrams orally every four hours for nasal congestion.
Phenylephrine is a decongestant that is used to treat nasal and sinus congestion, notes WebMD. It is also effective for treating inner ear conditions brought on by the common cold, influenza or allergies and breathing conditions such as sinusitis and bronchitis. It works by shrinking the dilated blood vessels in the nasal passages that cause nasal congestion, according to Drugs.com.