How Opium Addiction Happens:

Alkaloids found in Opium will build up in your system and stay there for 24 to 48 hours.

Opium consists of many chemicals that affect the human body. The most notable of the opium alkaloids are morphine and codeine. Both are narcotic in their effects, but morphine is the more potent of the two. When morphine, codeine, or any other opiate is introduced into the body, a host of things begin to occur. The first time user of opiates usually will become nauseous and even vomit. The pupils constrict, pulse slows, blood pressure falls, and respiration slows. The user begins to feel warm, euphoric, and relaxed. As time progresses, feelings of fear, panic, and anxiety lessen. Depending on how much has been taken, this progresses into a drowsy, dreamy state leading to a deep sometimes-dreamless sleep. Repeated use leads to more appreciation of the narcotic effects of opiates. However, repeated use also leads to addiction.

If the opiate is suddenly stopped, withdrawal symptoms will occur. These include restlessness, anxiety, rapid pulse, increased blood pressure, crying, runny nose, yawning, muscle spasms, back pain, tremors, and many other symptoms. These will usually begin 8 - 24 hours after the last dose, peak within 48 - 72 hours, and subside in about 7 - 10 days.
Tolerance occurs when the body gets used to a certain amount of the opiate being there, and it requires more and more to get the desired effect.