His later works may be described briefly as royalist pamphlets, written with more or less caution, as the times required. Drummond took the part of Montrose; and a letter from the Royalist leader in 1646 acknowledged his services. He also wrote a pamphlet, "A Vindication of the Hamiltons", supporting the claims of the Duke of Hamilton to lead the Scottish army which was to release Charles I. It is said that Drummond's health received a severe shock when news was brought of the king's execution.