Thank you for watching, really appreciate the feedback!

do you have a video about pharmacokinetics calculation step by step, like u said before?

Yeah if u got such vdo pls drop the link below

Hey sir may i know from where u belongs because am studying pharmD am indian so i have no idea about how the scenario of pharmD's graduates outside india so it will be great pleasure if you share some information. Thank you.

Great Explanation thank you WOW

Welcome

Phenytoin is considered to be a "mixed order" (aka second order, aka Michaelis-Menton modeled) drug, meaning that it behaves as a zero order drug at high concentrations (this is the way it's normally clinically administered) and behaves as a first order drug at low concentrations. Zero order drugs have a linear arithmetic dose-elimination curve, while first order drugs have a dose-elimination curve that is somewhat exponential-like. This means that first order drugs are eliminated from the body very quickly at first, and then at a progressively slower rate once the drug concentration approaches lower numbers (e.g., if elimination half life is 1hr, then after 1hr, only 50% drug remains). However, for zero order drugs, since they have a linear constant rate of elimination (e.g, if elimination is 20% per 1hr, then after 1h, 80% drug remains, and after 2h, 60% drug remains), it takes zero order drugs LONGER to be eliminated from the body compared w/ first order drugs (most drugs used in clinical practice). Thus, there's an increased risk for zero order drugs (e.g., 80% remains after 1h) vs. first order drugs (e.g., 50% remains after 1h).

Half life for 1st order drugs remains constant, while for zero order drugs, half life changes depending on the particular rate of clearance for each specific drug.

The drug passes through the intestinal wall into the portal circulation, then through the liver, then into the inferior vena cava, then to the heart, and then to the rest of the body. Some drugs pass from the intestine into the lymphatic vessels, and then into the heart without passing through the liver.

...so the correct answer should be the portal vein, right?

you r right. I think should be topical rather.

Topical route

From the time when the drug is in the system.

MC Ho Thanks. I'm still a little confused. By "in the system" do you mean when the first traces of the drug are detectable in the blood, or when all of the drug is in the blood? If a drug's peak concentration is 100 mcg/DL, and it reaches its peak concentration in 2 hours, and its half life is 5 hours, what will its concentration be at one half life and at two half lifes?

***** The simple answer would be when the first trace of drug enters the blood. As for the second question, as the definition of half life is the time elapsed for the plasma conc. of a drug to reach 50% of it's initial conc. (at t0), the conc. would be 50mcg/dL at first t1/2, and 25mcg/dL at 2nd t1/2 in the example. Not sure if that is the answer you are looking for. As you may know, the half life of a drug is dependent of a lot of factors (eg. route of drug administration(IV, inhalation, oral...), infusion rate, rate of metabolism, rate and volume of (re)distribution, rate of elimination...etc.)and is often time only obtainable by using computer simulation(eg. for multicompartment system) in form of a statistical distribution(different individual physiology). From what I understand, more applicable half life calculation would be in cases that more closely approximate single compartment system. Such as IV or inhalation drugs that have fast induction rate, with negligible time elapsed before reaching the peak conc., and minimal redistribution/tissue retention. I do not claim to be an expert in this topic. It would be interesting to hear opinions from the others.

Thanks for the comment! Zero order kinetics would be a linear graph, while first order would have a non linear graph. Sorry for the confusion!

yes the main organs for elimination are kidney and the liver.

Thank you for your generous feedback!

u doing phrmcY?