Listen to Trusbx folks. He does this stuff for a living and what he's said in this forum about medical issues can be counted on!!
I do "stress and anxiety" for a living..and I treat many trauma surviviors. There is NO DRUG worth using to "calm down" in these situations. The risk-benefit ratio just isn't there. Anxiolytic drugs may (may.. but I don't like them) be used to aid recovery. But NO DRUG (i.e. no mood-altering drug) is helpful in the immediate/acute situation.

The following is the Benson Breathing Technique. It is the (IMHO) most effective technique we have for overall calming. It can be readily learned, safely and effectively used, and it absolutely does work if done properly. (Trusbx, if you can monitor it, you'll see CO2 balance change after 2-3 minutes of this).

Benson Breathing (or Breathing Retraining)

It is important to understand that you are working to achieve a Calming Reflex, which you can induce with breathing and muscle relaxation techniques: with practice this reflex becomes more effective, and is more quickly induced.

The Benson Breathing (BB) technique is simply this:
You breath in, fully and gently.
You hold your breath, without tightening up.
You breath out fully, slightly pursing your lips as if blowing on hot coffee.

Make the three parts equal: IN HOLD OUT.
It usually helps to establish a count or cadence: IN.2.3. HOLD.2.3. OUT.2.3..

Yawning is a good sign. Slight lightheadedness suggests you haven't found a comfortable rhythm, so breathe normally for a while and try again.

Practice when you DON'T need the technique, 3 or more 10-15 minute sessions daily. Most people in my practice can produce substantial calming after 2 minutes of this, after a week to 10 days of practice.