It is better to wait for the day of your missed period before you take a test, because that is the time at which you are able to get the most accurate result. With blood tests, it is possible to get an earlier accurate result. If you used medications containing hCG, it will take around 10 days for that to leave your body totally. Taking a test before 10 or 12 days will result in a positive pregnancy test if you used hCG-containing drugs, simply because pregnancy tests work by detecting this very hormone.
You will still be able to take a test on the day of your expected period. But anyone who has been using medication containing hCG should be aware of a higher chance of getting a false positive, in other words a test showing that you are pregnant, when you are not. If you suspect you are pregnant, by all means take a test, but be sure to contact your healthcare provider to confirm that you are in fact pregnant if you do get a positive result.