Taking large amounts of strong medications on holiday

Not quite narcotics, but not far off. I'm hoping to go on holiday soon with someone that suffers from major depression and anxiety. She's on a daily dose of a strong anti-psychotic (not quite tamazepam, but not far off) and also takes anti-anxiety medication. Unfortunately these medications are regulated under most countries' narcotics acts, which could make for some interesting conversations at customs.

There are a few issues that I'm concerned with:

1. She can't go without. A single day without meds, or with a heavily reduced dose, can result panic attacks, manic episodes, complete catatonia, or even suicidal behaviour. This isn't temporary either - it causes medium term mental instability and can trigger a long term depressive phase. Missing a dose is potentially life threatening. This means we can't put all of the medication in her suitcase, in case it gets lost or delayed. She'll have to take some in her hand luggage.
2. This stuff isn't something you can get over the counter, and most pharmacies order it in instead of storing it because of the risk of junkies robbing them for it. I can't imagine it being particularly easy to get hold of abroad. If it ends up getting stolen or confiscated for whatever reason, getting hold of replacements might be problematic.
3. She is prescribed more than the standard dosage limit of one of her medications. This means that she has to bring a lot of the medication with her. This also might result in problems if she needs to be given medication in a hospital abroad. It also might raise questions at customs if anyone is familiar with the standard dosage.

Now, we can obviously get a doctor's letter to cover it, and they'll provide us with enough meds to keep her going for the time we're out there, but I'm not entirely sure that a doctor's letter from some random GP in England is going to be enough to convince an over-zealous border agent or police officer that we're not drug traffickers.

We're mainly looking at countries in the EU, but we're also hoping to go to Japan at some point. Any advice for how to handle this kind of thing? Are there any restrictions we're likely to run into?