Meth makes a person more awake and physically active, causes rapid heart rate and irregular heartbeat, and increases blood pressure and body temperature. It can overheat you so much that you pass out; sometimes this can even be fatal.
People who inject meth risk getting HIV/AIDS or hepatitis (a liver disease) if they share used needles. People also can get HIV by having unsafe sex. They may forget to use condoms because they're high on meth.