cool pack
Something like that. My son (21) is a type 1 diabetic. We have never had an issue with his insulin as long as it is not in direct sunlight. Something like the above link inside your pannier should do fine. Keep the cooling the gel pack as often as possible don't worry too much. Talk with your Dr and see what the max temperature and how long for your type of med. If you don't mind a little more bulky, most pharmacies get small Styrofoam containers with gel packs in shipments. They will usually give them away free. It would take up about half of a large rear pannier tho. Best bet is talk with the Doc for your particular med and follow his suggestion. It may just be it wont have a long shelf life, in which case you can get smaller doses and have some mailed in cool packs to different places on route.