The effectiveness of a Gilead Sciences Inc. GILD, +1.05% emergency treatment for COVID-19 could be hampered by other drugs that have been associated with treating the disease, the Food and Drug Administration warned late Monday. The agency said that remdesivir, a drug made by Gilead that recently gained an emergency use authorization from the FDA, could be adversely affected if combined with the drugs chloroquine phosphate or hydroxychloroquine sulfate. The FDA said that the latter two drugs could reduce the effectiveness of the Gilead drug. Earlier Monday, the FDA revoked its emergency use authorization for hydroxychloroquine and chloroquine. Gilead shares rose 0.1% after hours, following a 1.1% rise to close the regular session at $73.97.