John Gill Commentary Isaiah 38:21

"Now Isaiah had said, Let them take a cake of figs, and lay it for a plaster upon the boil, and he shall recover." — Isaiah 38:21 (ASV)

For Isaiah had said
Before the above writing was made, which ends in the preceding verse; for this and the following are added by Isaiah, or some other person, taken out of (2 Kings 20:7 2 Kings 20:8). The Septuagint version adds, "to Hezekiah"; but the speech seems rather directed to some of his servants, or those that were about him:

let them take a lump of figs, and lay it for a plaster upon the boil,
and he shall recover ;
which was done, and he did accordingly recover.

Aben Ezra, Jarchi, and Kimchi, all of them say, that this was a miracle within a miracle, since figs are hurtful to ulcers; and so say others; though it is observed by some, that they are useful for the ripening and breaking of ulcers; however, it was not from the natural force of these figs, but by the power of God, that this cure was effected;

for, without that, it was impossible so malignant an ulcer and so deadly a sickness as Hezekiah's were could have been cured, and especially so suddenly; nor were these figs used as a medicine, but as a sign of recovery, according to the Lord's promise, and as a means of assisting Hezekiah's faith in it.