The phrase "only medicine" is an idiomatic expression that means there is no other solution or remedy to a problem except for taking a particular course of action. Full definition
Not only the medicines, but the gentle solutions can give relief from the runny nose. (newbornbabyzone.com)
Every farm has a veterinary health plan and prescription only medicines are only given for specific reasons, they are never prescribed as a matter of routine. (waitrose.com)
Give your toddler only medicine prescribed for him and his specific condition. (babysafehomes.com)