Trip stoppers

Promethazine is the stronger brother of Diphenhydramine: Both are primarily histamine antagonists (makes you drowsy and less allergic to all sorts of stuff) with Anticholinergic (Belladonna-like) side effects. Antihistamines are useless against Trip anxieties.

Benzo(diazepine)s like Xanax DO work great & this is because they fortify the neurotransmitter GABA (Gamma-aminobutyric acid; the amino analogue of GHB) In order to have a "BenzoTrip"-like experience one has to take a comed that works on GABA the same way.

There IS such a thing; Legal, Cheap and Readily Available.
It is a form of Vitamin B-3 called Nicotinic Acid Amide aka Niacinamide aka Nicotinamide. This is not Nicotinic ACID aka NIACIN: this gives a bodyrush which is far from pleasant. A good health food store or vitamin supplier can get you a pot with 100 tabs of 100mg Niacinamide for about $ 5.
How much to take, then?

Let's say you'd want to take 1-2.5grams of Cubies, or about 10-12mgs of Psilocybin, this being a relatively good dose. (around LEVEL 3) To make this a "BenzoTrip" one has to add the equivalent of 1.25-2.5mgs of Valium, a tiny amount. This is where our friend B-3 AMIDE comes in!

"100mg Niacinamide equals 1-2mgs of Diazepam (Valium)"

For named Level 3 Trip you simply have to pop 100-300mgs of Niacinamide at the time of dosing up to experience the healthy equivalent of a BenzoTrip, this being a trip of reduced anxieties and body discomforts.

If you add B-3 AMIDE to a trip gone sour, expect a strong increase of effects within 5min and lasting about 15-30min before it gradually mellows stuff out.
You should NOT do this: in the rare case a trip should be stopped or mellowed, one simply cannot afford to intensify it. Hitting the brakes on a bad trip should only be reserved for REAL CRISIS EVENTS, but it's OK to pop a little B-3 AMIDE or a tiny piece of Benzo at the time of ingesting.

B-3 Amide should not be taken in amounts greater then 10mgs per kg bodyweight if it is to be used daily and for the once in a while Trip one can go to roughly 15-20mg/kg. Remember that if there's too much B-3 Amide or Benzo added to a Trip (like Valium 10 or a gram of B3 Amide on a Level 3) the trip will lose it's quality, potency or will stop alltogether IN MOST CASES.
1mg Flunitrazepam (Rohypnol) = 10mg Diazepam (Valium) = 20mg Oxazepam

Benzo's inhibit the effects of Major Psychedelics (Like Lysergide, but not THC) to an extent determined by dose. They do this so effectively that they are the #1 treatment against Bad Trips in European Emergency Medicine. They're so superior in this over Antipsychotics like Haloperidol that the latter's never given there.
(Which is PROOF that Psychedelic states in general ARE NOT Psychotic states!)
But besides stopping Trips off like nothing you've ever experiened there's another use for the combination, one that'll make quite some waves over here at the shroomery, for most trippers I can think of are bound to get very interested...

If you combine a certain dose of Mushrooms with a certain small dose of any Benzo, a Trip ensues which I have named: "BenzoTrip" or "Shielded Trip". This varies from person to person & you'll have to figure that one out yourself. I can however provide you with hints as to what could work for you, based on fearless self-experimentation.
Let's start with your mushroom. If you're into Psilocybe Cubensis you can roughly say that very good dried Cubes has ~12mg Psilocybin to a gram, while Cubes of average potency hang around 6mg/gram. (Based on good Dutch research) If both are taken together, these were the results from multiple self-experiments:

Psilocybin 10-20mg + 10mg Valium equiv. nearly completely PREVENTED the trip from happening. I was in a strange & way too mild "twilight zone" & Bonged up.

Psilocybin 10-15mg + 1.25-2.5mg Valium equiv. was just right: BENZOTRIP.

When I combined 30mg Psilocybin (a Whoopsie for me!) with 20mg Valium equiv. it came down hard on the trip but I retched & puked which was clearly Benzo.

Benzo's are slightly toxic and regular use likely leads to tolerance and dependence with possibly SEVERE WITHDRAWAL.