As parents drugs can be one of them difficult subjects to discuss.
At school your child will tend to cover topics like drug awareness under their Personal, Social and Health Education (PSHE) lessons in Year 5 or Year 6 depending on your school.
Its good to be honest with your child, allow them to ask questions about the topic if they are curious. It can be easily brought up into everyday life, if your child is sick and needs medicine explain to them why they need it and what it does, you can explain the benefits of these types of drugs. Keep the conversations light and calm so they don't feel scared or bombarded.
For more information on when is best to talk about drugs and how with your child visit these websites, or talk to your school and ask for resources/leaflets/websites that they might use that can help you at home: