Re: Nortriptyline
Nortriptyline is the milder active metabolite of amitriptyline. It's a good alternative to SSRIs if you're particularly sensitive to them.
I don't think you need to break them in half, as it's such a low dose, I really don't think you'll get any major side effects. Give it a go as it could really help - and you'll never know until you take them - so don't go leaving them on the shelf!
Even if you were to get mild palps etc it shouldn't be prolonged or intense and tbh there's more chance of your anxiety causing this effect anyway.
Let us know how you get on, and don't forget that looking into other forms of non-med therapies is also very worthwhile (self-help books, CDs, CBT, counselling etc).
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KK
Never Surrender, Comrade