The body might start producing more enzymes to break the substance down, and hormone levels could change. When use of the drug stops, the body fails to function normally because it has adjusted to compensate for the presence of the drug, and this abnormal functioning is felt as withdrawal. The higher the degree of tolerance and dependence, the worse the withdrawal symptoms will be. Withdrawal to different drugs varies in unpleasantness, severity and risk for death. Withdrawal from heroin is one of the most unpleasant withdrawals, but withdrawal from heavy barbiturate or alcohol use after a very long period of use has a greater risk of death.
Withdrawal symptoms are usually the opposite of the effects and side effects of the drug.
For example, one of the side effects of heroin is constipation, and one of the withdrawal effects from heroin is diarrhea. If you are using a drug repeatedly over a period of time, you will develop dependence on that drug. But you will also develop dependence to other drugs that you have not used that are in the same class as the drug you are using. This means that when you stop using one drug and begin to experience withdrawal, prescribed use of another drug in the same class might stop or decrease the severity of withdrawal. Cross addiction has also been used to describe addiction to more than one drug, whether or not it is in the same class.