That may depend on what type of mental illness we're talking about here and the severity of your relationship. If you're not serious, then forget about it. If you feel that you may be getting serious, then you need to tell him.
You don't want to surprise him or catch him off guard. Some experts would say that as early as a few dates in, you should divulge this information.
You know, casually slip it into your conversation about your childhood, obstacles you've faced, brief medical histories, and so on.
For instance, you could say: If I seem a bit emotional at times, I'm sorry. I'm bipolar and I'm on (fill in the blank).
Or: If I seem to be too distracted, I don't mean to be. I've been dealing with ADHD most of my life. Even with medication, I still have some things to work on.
Or: Yikes! Guess my Prozac hasn't kicked in yet.
Making light of the situation may help any discomfort he may feel concerning the situation. Still, being on medication is important and isn't something you should hide if you plan to continue the relationship. Nor should you stop taking them for his sake. He will have to understand. Fortunately, with all the medication commercials on tv, it's a bit easier to talk about certain illnesses and disorders. People may not understand these issues, but they are aware, so it's easier to start the conversation.
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