Personally, I don't buy cough medicine, but do use hot drinks eg lemon & ginger tea, sometimes with honey and / or whisky added (depending on driving / time of day etc) or gargle with hot salty water (particularly for a sore throat) or use normal boiled sweets (1 or 2 don't affect my blood sugar, but more may need more insulin). Obviously, everyone reacts differently to sugar and / or infections.
Might as well ask pharmacist (might be female, Caroline!) if / when you go to chemist to buy something.