If two medication containing the same active ingredient are considered ''bioequivilant'', it means that the rate at which they act (onset of action) and the extent of their action is within acceptable limits, and contributes towards ensuring the efficacy of medicinal products; this also means that these levels are comparable. It is important to note, however, that for certain medication such as lithium, it is crucial to stick to using the same brand prescribed and advised by the doctor, as these medication have a narrow therapeutic index and alterations in their plasma levels can be very problematic (Always speak to a healthcare professional if unsure). If we are talking about paracetamol, any brand regulated by a regulating body can be bioequivilant. The key word here is ''regulating body''. For example, a medicine bought in a European state would be regulated by the European Medicines Agency to ensure its safety, efficacy and quality. So as an example if you buy two different brands of paracetamol in an EU country, these regulations apply and they will both be safe, effective and of acceptable quality; however, if you buy medication from another country and are not sure about the standards of their regulating body, those may not have the same standards as medication purchased in an EU country. You can even be sold counterfeit medicine, which may not be safe for use, may not be effective and may not even include the active ingredient. So overall, if you buy two different brands of paracetamol in a country with a regulating body to ensure its effectiveness, safety and quality, they will work to the same extent and within acceptable limits to alleviate your symptoms; that would mean that to buy standard paracetamol over the counter for a simple headache, you can choose to buy the expensive one under a certain brand, or the cheapest one under a different brand! If ever unsure, it is highly advisable to read the product literature as well as speak to the appropriate healthcare professional.