Drug abuse is a major cause of hand tremors. When someone takes drugs, the chemicals go to the parts of the brain that control thoughts and movements. The brain cells become so damaged to the point where certain bodily functions are lost.
To prevent any further damage to the brain, get off the harmful substances immediately. Stopping cold turkey is always the best solution, whenever possible. If they cannot, they should at least ease their way into rehab. Some people become so addicted that their hands stop shaking after they consume more alcohol. If that is the case, they would definitely want to start in rehab first.
As one stops taking the harmful substances, one would almost immediately feel parts of the body improve dramatically and less inclined to run to empty the bladder. One feels less likely to sweat and show signs of nervousness. And problems with shaky hands will consequently diminish.