What is the ruling of Islamic law on leasing out a pharmacy, including the medications and medical equipment it contains, and is this permissible?

Renting a shop with its merchandise is not permissible, because ijarah (leasing/renting) is the transfer of usufruct (manfa'ah), while merchandise is a standing asset (ayn qā'imah). Therefore, renting it is, in reality, a sale. In contracts, what matters are the meanings, not the wordings.

Your friend can partner with you in the pharmacy where the capital is from you and the work is from him, and you share the profit, or agree upon a specific percentage.

As for what has passed, it is invalid (fāsid), and all the profit is yours, while your friend is only entitled to a fair wage (ujrat al-mithl).