Every neuro I have has px the 180 for me, but have also told me NOT to take it during the day because the time realease is not reliable. Possibly it's because I have generalized MG and breathing issues hit fast on me, when they do? Maybe they've worked out the problem recently and my neuro doesn't know/trust it?
I certainly wouldn't change whats working for you though! You may just ask him about it when you see him next - I'd be interested in knowing if he has updated info that mine doesn't

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Becky