Answer: Throat Coat contains a very large amount of licorice root (760 mg per tea bag) plus another 60 mg of a 6:1 licorice root extract and these ingredients are likely to be the cause of your issues. Licorice root provides great flavor, but you need to exercise caution and moderation with it because it contains a compound that can, through an effect on the kidneys to cause loss of potassium, fluid retention, increased blood pressure, abnormal heart rhythms, and lethargy. Throat Coat has a warning on its box regarding these issues, although the warning does not identify the causative agent — licorice root. See our article about licorice root for more details about its side effects, as well as the findings of clinical studies of its proposed uses.
Be aware that the FDA has warned that eating 2 ounces of black licorice a day for at least two weeks can cause similar problems.
Homemade licorice tea has been reported to cause similar problems. An 84-year-old man (with hypertension controlled by medication) developed extremely high blood pressure, headache, photophobia, chest pain (due to pulmonary edema) and fatigue, as well as low potassium after two weeks of drinking 1 to 2 glasses daily of homemade licorice root extract called "erk sous" (Falet, CMAJ 2019).A 57-year old man in Turkey with no prior history of cardiovascular disease experienced atrial fibrillation (a rapid, irregular heartbeat) likely caused by low potassium levels after consuming four glasses of "licorice root syrup" daily for one month (Erkus, Turk Kardiyol Dern Ars 2016).
Dietary supplements containing licorice root can also be problematic. A 68-year-old Chinese-American woman developed dangerously high blood pressure (219/123 mm Hg) resulting in a stroke (with symptoms including difficulty speaking and paralysis on one side of the body) after taking Chinese herbal supplement pills providing 800 mg of licorice root daily for two weeks to treat indigestion. The supplement contained several other ingredients, such as ginger root and cinnamon bark, but the reporting physicians noted the woman could have been consuming 8 times the maximum dose recommended by the European Commission Scientific Committee on Food of glycyrrhizin, the compound in licorice known to increase blood pressure (Shin, Neurohospitalist 2019).