This will be obvious to many, but looking through the threads many seem to think that you need to open your pip boy during battle to take a chem.

During a relaxing time go to your aid menu and go to your essentials like jet and water and you will see an option called favourite. Click it and do what it suggests, make a hotkey for it. On xbox you just assign them to your cross key. Then whenever you want to use them jt takes a fraction of a second to get those drugs into the veins, no need to open pipboy during combat.

One last tip, do not run out as it will uncouple it from the hotkey and you will have to do the favourite thing over again.