given someone else medication

Call your doctor and find out what the side effects are, and if you need to go to the Emergency Room!

Then, assuming you don't have to call the ER, call the drug store and let them know about the error as the same has probably happened to the guy in his 30s.

You really need to ask? Take it back to the drug store and demand the right medicine. Oh. Wait. Are you asking how you can sue the pharmacy for a million dollars? In that case, I guess the answer would be "see a lawyer". I guess you can't be all that sick or you would have thought of the first one anyway without having to ask on a web forum. It might be worth checking if the other person's medication is likely to have harmed you.

You know the person it was intended for? Maybe he got yours and you can just trade.

He needs to do more than just trade. Both the manager of the pharmacy and the manager of the store need to be notified of the mix up so they can do an audit of their procedures and fix the problem. The pharmacy will probably also have to list it in their regular reports to whatever agency they have overseeing them.

Something similar happened to my brother a couple years ago and he had such a severe reaction to the medication that he had to go to the ER. It nearly killed him. He didn't get another person's prescription, his prescription was filled with the incorrect medicine.