An adrenaline auto injector contains a life saving injection of adrenaline which needs to be administered if someone is having a severe allergic reaction. Depending on the type of reaction it can develop really quickly and cause someone to become very ill very quickly. If you are with someone who you know suffers from a reaction and you feel it is appropriate for the injection to be administered it is better for them to have the injection when they need it than not to receive it.
We produce adrenaline naturally so the worst that will happen if they receive it when it is not needed is that they will feel light headed. It tends to have a short half life so will leave the system fairly quickly.