It depends on the drug. Using drugs, again and again, for a long time can cause lots of major medical problems, from lung cancer (weed) to liver problems (alcohol) to brain damage (ecstasy, alcohol).

In other words, every drug is different, but all the long-term effects are bad. Besides the physical drawbacks, drugs cause major long-term emotional problems, like depression. People who are addicted to drugs usually up hurting the people they love. They lie, steal, even get violent. Their only goal becomes getting high.