Three things that I can think of off the top of my head cause "medicinal" flavors.
One is having chlorine or chlorinated water in your brew. Sanitizing with bleach, for example, or using tap water that has chlorine or chloramines in it will cause a medicinal flavor.
Another is using some yeast strains and fermenting above 70 degrees or so. Some yeast strains get really "clove-like" which can medicinal.
The last is bacterial contamination. If you sanitized well, that shouldn't be an issue.
I would guess that either the water or fermentation temperatures (or a combo of both) is responsible here.