There is no such thing as a "one size fits all" product
Recognising the type of calming product you require is the most important step - because not all products are suitable for all horses. We also need to recognise that some horses do not respond to any supplement solution at all.
Spooky - sees things that aren't there. Hears things a million miles away. Overreacts to situations. Then consider Cavalor SoZen.
Hot / Sharp - For most horses a magnesium based supplement is the answer and the use of magnesium is scientifically proven to work. Check out Cavalor Calm for regular use or Cavalor Take it Easy for the most stressful situations.
Mare or Stallion Behaviour - Cavalor Venus is your product.
Emergency! - Then carry a tube of Cavalor Take it Easy Forte