How do I tell my PhD advisor that I'm about to start medications that impair my cognition?
In the UK, disability legislation says an employer has to make 'reasonable accomodations' for long term health issues like yours. Mental health problems are (sadly) not uncommon in academia. It sounds like you get on well enough with your supervisor that you could just tell him what's up. If you're not comfortable with that, you might consider going through more official channels.