Are you asking how much is a lethal dose? It is very hard to determine the minimum lethal dose for methamphetamine. It is highly variable and depends on many factors, such as the route of administration, the person's tolerance to meth, how often they use it, whether or not they have any other health issues, their unique brain/body/metabolism, any other drugs they have taken, etc.
An overdose is not always lethal, "overdose" just means that the person took the drug in quantities greater than are generally considered safe for them and may result in serious side effects, toxicity or even death. A fatal overdose is usually characterized by a rapid onset of physiological deterioration, eventually leading to organ failure, heart attack or stroke. When death occurs, it is often very sudden.
Symptoms of an overdose can include: sweating profusely, rapid breathing, increased heart rate, abnormally high blood pressure or abnormally low blood pressure, changes in heart rhythm, high temperature, nausea/vomiting/diarrhea, very dilated pupils, coma, shock, inability to urinate, muscle twitching/tremors/seizures, hyperthermia, confusion, delirium, weakness or hyperactivity. Of course having some of these symptoms does not necessarily mean that the person has taken a life-threatening overdose, nor does one need to have all of them to signify a serious overdose. Keeping hydrated is very important and keeping cool if the body temperature is too high.
Were you wanting to know at what point one should call an ambulance? If you believe you or someone else may have taken a serious overdose it's best to seek medical attention immediately. If the person has symptoms of shock, inability to urinate, and muscle twitching/tremors/seizures, or coma, definitely call an ambulance right away.
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