To understand these lines, consider them in the context of the whole passage:

Well, march we on.

To give obedience where 'tis truly owed.

Meet we the med'cine of the sickly weal,

And with him pour we, in our country's purge,

Each drop of us.

In the scene, Caithness speaks to Menteith but refers to Angus, Lennox, and their other soldiers, as well as to himself and Menteith. Malcolm is not present with them at this time. Caithness and the others are preparing to join with Malcolm's forces and attack Macbeth at Dunsinane.

Caithness says that they will "give obedience where 'tis truly owed," to Malcolm the rightful heir. It is Malcolm who is "the med'cine of the sickly weal," the ruler who will heal a suffering Scotland after Macbeth's reign of terror. Caithness says that they will join Malcolm and fight with him for the restoration of Scotland with their very last drop of blood.