Earlier than 6 weeks the injection is likely to provoke heavy and prolonged bleeding. However, DMPA can be given earlier than six weeks depending on the situation with advice about the risk of bleeding. Antibiotics do not pose a risk to DMPA injection contraception. Liver enzyme inducing drugs such as rifampicin may require the injection to be given 2 to 4 weeks early. The DMPA inject seems to have no affect on other drugs.
Most women have less or no bleeding or period pain and less or no premenstrual tension.
On balance DMPA reduces the risk of cancer. The main problem with DMPA is the drug once injected cannot be removed and is active for 2 to 3 months. Dr Bill's - Home Page.