You cannot delete a medicine, but you can inactivate it so that it no longer appears in your selection list when entering a disease. 

It can be inactivate by choosing settings animal registration at the top right of below your name.

Here you will find all medicines that are in your system under 12. Diseases/ treatments

By clicking on the pencil behind the medicine you can change the medicine, on the right you can put it on active or not active.

Click save to save the change.