The onset of withdrawal symptoms can vary depending on the substance being used, the frequency and duration of use, and individual factors such as metabolism and overall health. Some drugs, such as heroin, can cause withdrawal symptoms within hours of the last use, while others, such as benzodiazepines, can take several days to develop. Additionally, the severity of withdrawal symptoms can also vary widely, ranging from mild discomfort to life-threatening complications. It is important to seek medical attention if you are experiencing withdrawal symptoms or are planning to quit using drugs or alcohol.