to feel he should have told me about this health condition before we married

There are links to autidsm with some epilepsy meds for both parents.

Maybe it is. Makes him a bit of a crap person though. Lying by omission is not a symptom of epilepsy.

Yes, anger is directly related to epilepsy.

Sudden feelings of anger or irritability can be caused by the condition itself—especially during or immediately after a seizure—as well as by the emotional stress of living with the disorder or as a side effect of anti-seizure medications.

The relationship between epilepsy and anger manifests in a few specific ways:

Focal Seizures: In rare cases, a seizure (usually originating in the temporal or frontal lobes of the brain) can manifest as a sudden, intense wave of anger or fear. These are called focal emotional seizures.

Post-Ictal State: It is common for people to experience irritability, aggression, or mood swings in the moments or days directly following a seizure, as the brain recovers.

General Triggers: Living with an unpredictable condition can cause significant frustration and anxiety, which often translates to anger or emotional outbursts.

Medication Side Effects: Certain anti-seizure medications are known to cause mood alterations, including sudden irritability and anger

I had epilepsy as a child after chicken pox went into the lining of my brain...out grew it thankfully.

I knew about post seizure irritability but none of the rest. Certainly your DH could get help for his anger OP

YANBU. He should have told you. Something so hugely important. Even though I didn't know i had it at the time I still told my husband that genetic Muscular Dystrophy is in my family and he has supported me through the whole testing, diagnosis etc.of that.
Even if he was managing it and had it under control he still should have told you. Ask him why he felt the need to not tell you about it, how can you have a full relationship with such huge secrets, what else might he be hiding?

Are you sure that this isn't an example of 'and another thing' if you are beginning to doubt whether you want to stay married? Because it's a long time after the event.

Perhaps your last paragraph is why he didn’t tell her 🤷🏻‍♀️. He was worried she’d leave. I understand that.

Bi-polar isn’t the same as epilepsy though.

I have to say, your understanding or lack of of epilepsy is showing through your posts. He was wrong not to tell you but the issues with his personality are probably not a result of his experiences of seizures or anything else. He’s possibly just an angry unsympathetic short fused person,

It could be entirely unrelated to his epilepsy! He could be just a bit of a shit person.

My sister died as a result of an epileptic seizure when she was 19. I’ve had seizures where I’ve injured myself (and knocked my front teeth out). I’m not an angry or unsympathetic person.

I'm 47, my best friend's husband has epilepsy - he campaigns at the House of Commons and was on the news last year being interviewed on the changes to PIP. No one in our circle has ever said any religious nonsense around it. And if they did he would very much set them straight. You'd have to be living in a very insular society for demonic possession to be taken remotely seriously in the latter half of the 20th century, when we were growing up, let alone today. My dad is 80, he wouldn't see epilepsy as a moral failing either.

Most epileptics who manage their condition sensibly have medical jewellery and carry cards that explain what medication they are on and how they would like to be helped if they have a seizure. If you collapse in the middle of the street, mum and dad aren't going to be there, are they?

Someone in their thirties should very much be taking control of their own condition and not relying on older relatives. In the ideal world, he would have disclosed this at the point of getting serious with the OP and then she could have decided if she wishes to continue the relationship and take on some responsibility for helping him manage his condition.

He should have told you but I get why he didn't. He saw it as his thing and didn't realise it had any bearing on you.

Are you sure he's prescribed it for epilepsy and not bipolar disorder? In any case, he withheld vital health information from you and that has profoundly damaged the trust in your relationship. Not to mention the whole thing with his parents is ridiculous.

Please tell me you didn't conceive while he was still on this medication.

@RachelWardd has this been a theme throughout your marriage? Are you through the menopause or in peri? Things can fester during the menopause.

How old are you? I'm nearly 60 and epilepsy was seen as a moral failing. I don't know if it was the 'speaking in tongues' etc from the Bible, but epilepsy was looked upon as a punishment from God, or demonic possession.

I'm thinking that the parents might have asked to support him with medication, for their own anxiety, but also because having a seizure and head injuries affects the memory. So, if he had to ever go to hospital, they know what meds he has been taking and when. There'd be no chance of overdosing etc. Head injuries (my DP has one) are still misunderstood and minimised.
OP, after 20 years, you should have got out of the marriage or got over it completely.

If this is/was a deal breaker/issue for you why didnt you leave him at the time?

But vitamins doesn't make sense either - why would he need to go somewhere daily for vitamins? As an adult?

But by the by - you knew his condition in 2008? And nearly 20 years later you are still ruminating on it? What do you want now - truly? Because you can't reverse time. You sound like you are eating yourself up over this.

It sounds as if you’re known about it for a while if he stopped taking meds in 2008?

It sounds more like his anger and selfishness (??) that is an issue? I am not sure you / he can blame epilepsy for that.