The tincture taken by Thomas throughout the first half of the movie is likely laudanum. Laudanum, or tincture of opium, is comprised of opium powder and alcohol. It is implied in the opening scene that his addiction is why he is thought to be dead prior to being located to help with his sister's rescue.
It had its world premiere at Fantastic Fest in September 2018, and began streaming on Netflix on October 12, 2018.