With most activable items e.g Horse Whistle, poisons, teleportation shards etc... you can drag and drop them from your bag into a hotbar, effectivly giving you a load more activable shots. Personally I have a whole hotbar dedicated to this with all my needed activeables on it
Also I have found that with "useable" items such as poison if you have more than one stack in your inventory the hotbar button automatically switches from the empty one to the next full one in your bags. This is great for any item that gets used up alot as you don't have to keep making a new hot button each time
When you use all the poison (or whatever) the button greys out and relights when you buy/add more into your bag.