Dr Alun Hughes
there are two phases of drug metabolism
phase 1 - works to either unveil or add a reactive group to a molecule this makes very reactive products
phase 2 - it adds long chained molecules onto these reactive groups to deactive them and make them not reactive anymore, the main function of phase 2 metabolism is to excrete the waste product/drugs from the body
it can do this by - hair, sweat, feces, urine, breath, milk, etc etc
bioavailability is the amount of a drug that is still in its original active form in plasma after being absorbed, each drugs bioavailability is compared to what it would be if inserted via IV this is because IV allows for 100% bioavailability due to no prior metabolism.
toxicity of drugs can occur when a drug goes above its therapeutic index value,
some drugs must go above this value inorder to be effective such as chemotherapy drugs because they must target cells of the same optimum values as those that are healthy cells within the body. if a person takes too many doses of paracetmol (As an example) it can cause a summative like event to take place and each new dose can build upon the last that was administered which can cause toxicity (outwith the therapeutic value)
some drugs such as statins have a very narrow therapeutic range and they are degraded by Cytochromes through metabolism both in the liver and the gut, this amount of metabolism allows for the perfect concentration to enter within the plasma concentration. if grapejuice is taken with statins, the grapejuice acts as an inhibitor to the cytochromes which metabolise the statins and this resukts in a higher amount of bioavailable statin concentration within the plasma which can be toxic to pateints
in regards to warfarin and phenobarbital.
if phenobarbital is taken with warfarin it increases metabolic effect of cytochromes which usually break down warfarin and this can cause warfarin to not be able to reach its therapeutic value which means it is not effective in its treatment