You can still use the kit if you are sick or on medication the day of collection.
However, if medication and/or illness results in watery stool and/or diarrhoea then it is advised not to use this kit. Stool sample collection is rather difficult with watery stool and can increase the chances of over/undersampling. Too little or too much of a stool sample can lead to an invalid test result and/or a rejected sample.