Anyone had any success with CIO? Any tips/advice?

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This is a sign of desperation. Before I was fully against CIO but now I'm quite literally on the brink. Been to the doctors this morning and ruled out anything medically wrong with him, it's down to 'attachment' and honestly CIO seems like the best option right now. So has anyone had any success stories to spur me on? Any advice?

How old is he? I’ve never used cio but I did sleep train all of mine. When I worked as a nanny I worked alongside the maternity nurses who sleep trained all the children.
Can you tell me what his routine looks like? What time he wakes? What time he falls asleep? How many naps he takes and at what time? How does he fall asleep? How many times does he wake up? Is he waking up to feed?

Can you clarify what you understand CIO to involve? There are many other gentler methods of sleep training. Have you already tried controlled crying, gradual retreat, anything else?

CIO gets short shrift on here - and personally I'm not sure why it would ever be a first resort, but it's not clear if you've tried other methods.