My advisor said he has to mention my mental health condition in recommendation letters. How much can he say?

I would like to know how much he can write about my mental health, also considering that the academic work didn't suffer at all from this.

According to some of organizations: "The Americans with Disabilities Act recognizes that employers may sometimes have to disclose medical information about applicants or employees. " But also they claim: "The basic legal principle that employers should follow is not to reveal medical information about you unless there is a legitimate business reason to do so. But because that standard is fairly vague, there are laws which more specifically protect the privacy of your medical records, such as the Americans with Disabilities Act, the law which makes it illegal to discriminate on the basis of an employee's disability."

For UK you may find more information on NHS website and Equality Act 2010 Guidance for employees.

If you ask me I would say that it is not actually ethical to write about your mental health if your job didn't suffer from that. For some careers there are some health restrictions (e.g. pilots, surgeons, etc.) but I've never seen that for academic work. More to say, I know people who have GAD, OCD, etc. and they have never had significant issues in academic life. But I also have to say that people usually don't take year off because of mental health issue, especially not for GAD. From your post I can't see if the GAD is a main reason of one year off. If it isn't the main reason then I don't understand why your supervisor has a need to write it in recommendation letter. But, if GAD is your main reason why you needed to stay away from University than I don't understand how your mental health didn't affect your productivity when it is even though you continued to work. Whatever the case is, I wouldn't like that my supervisor writes "a mental issue" because for me it sounds serious and Generalized Anxiety Disorder actually doesn't sound due to prevalence "GAD affects 6.8 million adults, or 3.1% of the U.S. population"

Shortly, I don't think he should write about your mental health when you obviously continued to work. It is your private information, there are some laws about it and I am worried that you will be discriminate because of that.

My advice: Talk with your supervisor, find out why he thinks that he is forced to write it in recommendation letter (that is really strange) and please look more into the laws and send them to him if it is needed.

If you received significant special accommodations while doing your PhD, e.g., double-time on your exams (especially comprehensive written exams and such) and other special help during your research period (was he easier on you and gave you more guidance and answers and harder on other students?), I am guessing that he feels that he must write a "fair" letter of recommendation, stating how you received special help -- and as a result of the special help, you were able to be a productive student and researcher. I highly doubt he will say much more than this, because your privacy is likely protected by several different laws.

I'm really not sure, though -- so this is just something for you to consider.

There's probably a misinterpretation going on. I suspect your advisor wants to explain something to the people reading it, rather than "warn" them. Something like:

he took a year off because he was ill

is okay to say.

you should be aware that he has condition X

may actually be illegal to say, and I would not want to receive such a letter. The people receiving it are suddenly at risk for a discrimination case if they do not give the position and it can be credibly shown that the reason was the disability.

So - ask your supervisor why he feels he needs to mention it. In parallel, contact your university's disabilities office to get a response. If you are uncomfortable with your supervisor's reason, tell him the advice you got from the disabilities office.

To give you an idea of how seriously some universities take various regulations - I had to have permission from an undergraduate to include their grade in a letter of recommendation.