The sooner you stop using the less risk you run of harming your baby. If you are nursing a baby it is especially important to be aware of whatever you ingest, as much of it travels through your bloodstream into your milk. Some substances such as caffeine do enter your bloodstream but appear in breast milk in such low levels that you would have to consume very large amounts for it to affect your baby. A balanced diet and lots of rest are vital to maintaining your health and providing your child with the right vitamins and nutrients. It is possible that using drugs may interfere with this.
Some women choose to wait until they are no longer nursing to resume drug use.
This can be tricky to judge however, as it is difficult to know how long a particular drug remains in your bloodstream. However, a drug using lifestyle may reduce the effectiveness of the pill as it is important to take it every day at the same time for consistency. This does not mean you should stop taking the pill on the days you are going to use drugs. If you do so, you will not be protected against pregnancy. Remember too, if you vomit or have diarrhea the pill may not work.