An injection of “depo” or “the shot” is protective against pregnancy for up to 3 months. It is given via injection into muscle usually within the first 5 days after starting a period. As it contains a progestin hormone only it is likely to cause irregular bleeding for the first 2 to 3 injections
However, afterwards most women will not have any menstrual bleeding for the remainder of usage. The shot may decrease bone mineral density yet this decrease in bone density is reversible once injections are stopped.