Most American and European drugs are available in Italy, even if they may be sold under a different name..
A pharmacy may not always have the drug requested but another pharmacy might have it. You can also ask the pharmacist if it’s possible to order it and pick it up later – in the afternoon if you ask in the morning, or the next morning if you asked in the afternoon.
If the pharmacist says the drug prescribed is no longer produced or cannot be found, you can ask to check the “informatore farmaceutico” or try with the Vatican drugstore near Saint Peter’s – via Porta Angelica, entrance Porta Sant’Anna.
If you should decide to go to the Vatican drugstore, remember to take the prescription with you as well an ID or Passport or you will not be able to enter.