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Irregular periods and prolonged spotting are very common side effects of Depo use.As it is a hormonal preparation woman may exhibit some changes in the body and present as cycle changes,nausea,vomitting,fatigue,cramps etc.These effects are usually present for initial few months and then subside as the body gets adjusted to it.
You can wait for few weeks and if it does not subside then visit a Gynaecologist for evaluation.
Take Care!