People will come and ask you for medicines by name, but you still need to make sure you ask them some questions. As always, check who the medicine is for, whether they have used it before and that it's the right choice for their symptoms.
You can add the questions in when dealing with the customer at the till, as well as offering suggestions and advice. For instance, you can point out the cautions on the pack while you are handing them their change and you can also suggest that if the problem isn't getting better, they should talk to the pharmacist about it. You can finish the conversation by asking them whether they need or would like any more information about their medicine.