Medicinal off-flavors are the result of using chlorine-based sanitizers, which react with yeast phenols during your brewing process. The best way to prevent these off-flavors is to rinse your equipment in boiling water following sanitation.
Metallic off-flavors are caused by unprotected metals dissolving into the wort, especially when using Iron and Aluminum pottery, which can leach into the wort during the boil. Stainless steel pottery will not cause this.
A secondary cause is the use of tap water in fermenting your beer; High chlorine or Iron content, along with the use of softened water with salt or gypsum, will also result in medicinal and/or metallic off-flavors.