It will help to split this into 2 questions:
- do I need a medical certificate?
- if I need a medical certificate, how current must it be?
If you can answer No to all of the standard PADI medical questions then you will not need a certificate. You will have to fill out a form with the NOs. Some places will then require a doctor to endorse your NOs. Other places trust your NO answers and let you dive without a doctors endorsement (similar to the model used by many US dive ops).
If you answer YES to any question then you will need a medical cert. It is good for 2 years until you turn 40 then it must be annual.
Best bet is to check with your dive operator to see how they interpret the rules.