Your baby's early development (even before you realise you are pregnant) is crucial for your baby's well being and further healthy development. If you are taking any medication, it may actually be harmful to your baby or may even interfere with the process of fertilisation without you knowing about it.
To prevent this from happening:
If you are taking medicines because of an illness, discuss it with your doctor that you are trying for a baby. Your medication may need to be adjusted.
Don't buy medicines blindly over-the-counter. Even if you have to, discuss it with the pharmacist first.
You don't really need any supplements (other than folic acid) if you are healthy and fit, so resist the temptation to buy them, unless advised by the doctor