From the CAA website:
All other hypnotics, including Zolpidem, Zopiclone and “over the counter” preparations such as Diphenhydramine and Promethazine are disqualifying for medical certification.
The way I read that is that if it is an ongoing prescription then it is disqualifying. However if you have taken it in the past but are no longer taking any medication related to your mental health then your AME is likely to be able to work with you on a way forwards. However, any past treatment for that subject would still need to be declared on your application.
Sadly since the GermanWings crash the whole industry is paranoid about allowing another occurrence. It doesn't mean you'll never get a medical, but don't expect an easy ride.