Hydroxychloroqine (C18H26ClN3O), also known as hydroxychloroquine sulfate and HCQ, is a drug sold under the name Plaquenil Prior to COVID-19 it was most commonly used to treat malaria, and also currently to treat lupus. The package insert for Plaquenil describes its mostly minor potential side effects some of which occur at much higher doses than what is used to treat COVID-19.
It has been used by the physician Dr. Vladimir Zelenko as a part of successful treatment for coronavirus and was recommended for such by President Trump (see: Vladimir Zelenko's coronavirus treatment). Specifically, Dr. Zelenko used hydroxychloroqine, azithromycin (an antibiotic) and zinc as a part of his treatments to his patients.
China, South Korea, and India all use hydroxychloroquine to successful combat the effects of COVID-19.
Never-Trumpers in government and hospitals block early use of this medication, and instead either withhold it from patients entirely or delay it until the end stage of the patient's life when medications are least effective. Other governments (such as Texas), wanting to make sure current patients had a supply for their pre-existing medical needs, permitted it only with a positive COVID-19 diagnosis.