Metanec was a painkiller. They were non-narcotic, and kept you both painless and awake. Its non-addictive nature was the reason that Rex Matheson was taking it following what should have been a mortal injury at the start of Miracle Day.
A PhiCorp warehouse on Boston and 8th Street was full of millions of bottles of metanec. The company was planning on using them all for the victims of Miracle Day, as such a drug was in great need. They had been planning this for at least a year, and had stocked up beforehand. (TV: Dead of Night)