The effectiveness of a drug depends upon it's dosage. How effective a drug will be in your system depends on how much of the drug you have taken. All drugs are available in different dosages because it's hard to get the balance just right for most people at first as to what exact dosage of a drug they will need this is because we are all different biologically, so what would be a correct dosage for one might not be enough or too much for someone else.
