Variability in testing and data collection methods makes comparisons of prevalence and progression difficult.The prevalence of myopia has been reported as high as 70–90% in some Asian countries, 30–40% in Europe and the United States, and 10–20% in Africa. Myopia is about twice as common in Jewish people than in people of non-Jewish ethnicity. Myopia is less common in African people and associated diaspora. In Americans between the ages of 12 and 54, myopia has been found to affect African Americans less than Caucasians. Asia
In some parts of Asia, myopia is very common. Singapore is believed to have the highest prevalence of myopia in the world; up to 80% of people there have myopia, but the accurate figure is unknown. Chinas myopia rate is 31%: 400 million of its 1.3 billion people are myopic. The prevalence of myopia in high school in China is 77%, and in college is more than 80%. In some areas, such as China and Malaysia, up to 41% of the adult population is myopic to 1.00 dpt, and up to 80% to 0.5 dpt. A study of Jordanian adults aged 17 to 40 found over half (54%) were myopic. Some research suggests the prevalence of myopia in Indian children is less than 15%. Europe
In first-year undergraduate students in the United Kingdom 50% of British whites and 53% of British Asians were myopic. A recent review found 27% of Western Europeans aged 40 or over have at least −1.00 diopters of myopia and 5% have at least −5.00 diopters.
Dependence and withdrawal
Drug tolerance develops rapidly in amphetamine abuse (i.e., recreational amphetamine use), so periods of extended abuse require increasingly larger doses of the drug in order to achieve the same effect. According to a Cochrane review on withdrawal in individuals who compulsively use amphetamine and methamphetamine, "when chronic heavy users abruptly discontinue amphetamine use, many report a time-limited withdrawal syndrome that occurs within 24 hours of their last dose." This review noted that withdrawal symptoms in chronic, high-dose users are frequent, occurring in roughly 88% of cases, and persist for 3–4 weeks with a marked "crash" phase occurring during the first week. Amphetamine withdrawal symptoms can include anxiety, drug craving, depressed mood, fatigue, increased appetite, increased movement or decreased movement, lack of motivation, sleeplessness or sleepiness, and lucid dreams. The review indicated that the severity of withdrawal symptoms is positively correlated with the age of the individual and the extent of their dependence. Mild withdrawal symptoms from the discontinuation of amphetamine treatment at therapeutic doses can be avoided by tapering the dose. Overdose
An amphetamine overdose can lead to many different symptoms, but is rarely fatal with appropriate care. The severity of overdose symptoms increases with dosage and decreases with drug tolerance to amphetamine. Tolerant individuals have been known to take as much as 5 grams of amphetamine in a day, which is roughly 100 times the maximum daily therapeutic dose. Symptoms of a moderate and extremely large overdose are listed below; fatal amphetamine poisoning usually also involves convulsions and coma.