Try to make sure that you have enough of any prescription medications you may be on to see you through at least two weeks. You'll need to speak with your pharmacist about this and let them know that you are wanting to have some of your medication in advance to be kept as spares.
Your doctor will need to be on board with this as well but again, if you explain what it is you are wanting to do, they should be okay with it.
Just make sure that you rotate them as you renew your prescription. You don't want to end up with out of date meds in your medicine cupboard.