Let’s hope you are really using the weed for medical reasons here. Then again, there isn’t really any lasting harm from smoking weed. It’s practically impossible to overdose on one, but very possible to get too high. Taking too much often leads to the dizziness, nausea and panic attacks, so you might want to get that dosage in check to start with.
With that said, taking a cold shower is one of the best ways to come down quickly. In theory, by running yourself under ice cold water, the blood would rush to the vital organs and wakes you up. In practice, often the cold itself is enough to numb yourself from the paranoia. This is the same method smokers and druggies used when going “cold turkey” (although weed itself is not addictive), but it does work wonders to calm yourself down.