I would not take it without symptoms or being tested as positive. You never know what sort of side effect a drug may cause an individual. Why tempt fate? Do not do it, Mr President.
Aspirin has the same side effects as hydroxychloroquine.
From Wikipedia:
Hydroxychloroquine was approved for medical use in the United States in 1955.[2] It is on the World Health Organization's List of Essential Medicines, the safest and most effective medicines needed in a health system.[6] In 2017, it was the 128th-most-prescribed medication in the United States, with more than five million prescriptions.[7]
Trump's job requires that he be around a large number of people. He is at risk, and the country cannot afford for him to be infected. If Trump's personal physician sees no contraindications, it makes sense for him to take the drug for prophylaxis.