I've taken promethazine many times over the past 10 years, for sleep induction and reduction of nausea, for which it can be very effective. I have misgivings though which I will share.
Its not going to replace a benzo, in terms of reducing your anxiety, but may have some benefit. I have used it for such, and it does 'work', to an extent. It just slows things down; it has antipsychotic properties (and can cause all the negative side effects associated with traditional, old-school antipsychotics) which may explain that, plus the sedation from the antihistamine aspect would contribute. Bear in mind, this aspect which effects dopamine can cause severe restlessness (akathasia), trembling, issues with swallowing, and difficulty talking/enunciating words; I've taken highish doses (in the past, to potentiate codeine and to reduce the itch associated) and have experienced stuttering and EXTREME mental confusion; literally, the inability to actually think, to say nothing of the inability to communicate. This is highly unpleasant, and has caused me pretty severe anxiety. That was at very high doses, from 150mg to 250mg, and the effect could last for several days. But I get similar twitching and trembling from prescribed doses now, so be wary.
Whilst it can help anxiety, its also a strong anticholinergic so a high dose can cause delerium and will cause dry mouth/sinus, dry eyes, decreased sweating (which can also manifest as sensations of heat/flushing), sense of derealisation/depersonalisation (which can cause anxiety) and dizziness/vertigo. It causes parathesia (pins and needles) which I associate with anxiety and benzo withdrawal, so that feeling often leads to a feedback cycle of physical anxiety.
It will help you sleep intially, though drowsiness can continue into the following day. I used to take it every night, 75mg for sleep, and did so for about 2 years. I decided to stop taking it, because it was giving me heart palpitations and confusion, and went through a sleepless period of 15 days (3 hrs maximum, extremely lucid dreams which was actually fucking awesome) and then several months of early waking, which I actually liked and so have continued. I have always been a poor sleeper, and this stuff really fucked with that for me. Its effective against mild nausea, but not 100%. The side effects can make the nausea preferable.
![:\](https://bluelight.org/xf/BL_Images/Orig_Emoji/wrygrin.gif "wry grin :\")
In summary, I think this drug can be used safely for medical reasons, but has a tremendous host of side effects, and is probably bad for you. It certainly causes sleep-dependance, and has detrimental effects on memory formation and learning. But, it has been helpful to me, and may be for you too. I would advise having no more then 25mg by day (if required) and maximum of 50mg for sleep, and I urge you NOT to take this often. It is safer then benzo's by far, but is by no means innocuous. It is a very powerful substance. It is also extremely unpleasant, and I would avoid it or use it sparingly.
