The effects of the drug begin 10–40 minutes after ingestion, and last 2–6 hours depending on dose, species, and individual metabolism. The half life of psilocybin is 163 ± 64 minutes when taken orally, or 74.1 ± 19.6 minutes when injected intravenously. A dosage of 4–10 mg, corresponding roughly to 50–300 micrograms per kilogram (µg/kg) of body weight, is required to induce psychedelic effects. A typical recreational dosage is 10–50 mg psilocybin, which is roughly equivalent to 10–50 grams of fresh mushrooms, or 1–5 grams of dried mushrooms. (A Johns Hopkins study found the ideal dose for long-term positive effects to be 20 mg per 70 kg of body weight.) A small number of people are unusually sensitive to psilocybin, such that a normally threshold-level dose of about 2 mg can result in effects usually associated with medium or high doses. In contrast, there are some who require relatively high doses to experience noticeable effects. Individual brain chemistry and metabolism play a large role in determining a person's response to psilocybin.