Many substances can cause withdrawal. Withdrawal is group of bad feelings that happen when someone stops taking a drug suddenly, if they were regularly taking that drug before. For someone to have withdrawal, they must be dependent on the drug. Different drugs cause different things to happen during withdrawal. They can also cause different amounts of trouble for the person in withdrawal. Withdrawal for some drugs, like heroin and other opiates, can be dangerous or deadly, and should be done with a doctor or nurse taking care of the person in withdrawal.