This clip is the next following "Pharmacist- Exploring the Problem". This situation shows the pharmacist prescribing medicine based on the consultation (talking to the patient about the problem) he has already done.
In this clip, it is now the customer who asks most of the questions. These are about the advice that she has been given by the pharmacist.
"What...?" "Will I...?" "Is that...?" "Are you sure...?"
The pharmacist reassures the customer by saying "trust me".
The pharmacist uses the expression "They'll block you off pretty quick" meaning that the medication will stop the diarrhoea very quickly. The word pretty is used informally to mean "considerably", therefore meaning very quickly.
"You must come straight back." "Straight" refers to the fact that if the medication does not succeed in working, the customer should return to the pharmacy immediately.