There are no guarantees. The increased dose may not work, but the odds are in your favour.
The recommended dose range for insomnia is 25-50mg. There isn't one for anxiety as this is an off-label use, but for most of its other indications the recommended daily doses are in the 6.75-75mg range and I don't see why this should be any different for anxiety. Promethazine has a fairly short half-life - 10-19 hours - so taking it once a day may not give adequate 24 hour relief.I didn't think I could have a dose increase on promethazine. I thought 25mg would be the standard dose.