Pharmacokinetics question?

Question

I have 2 questions.

Q1: What is divided dose in medicine?
For example it says: Initial dose 20 mg orally once daily in the morning; maintenance: 20-60 mg orally daily (single or divided doses) (maximum 80 mg/day).
Question for maintenance dose: 60 mg divided dose? how and when? Does it mean in 2 parts/3/4/? parts? Can it be morning=20mg, lunch=20mg, dinner=20 mg? Please tell me the definition of divided dose with an example.

Q2: How I can determine how often to administer a drug based on pharmacokinetics? For example it says:
- Absorption, oral: time to peak concentration 7h
- Elimination half-life = 12 h 
(It says also that in general it can be administered once daily because it has a 24 h effect.Why?)
- Can you give another example to clarify it better please?
- When it says divide in 2 doses , does it mean only in the morning and dinner? 3 doses (morning-lunch-dinner)?

Thank you for taking the time to answer!!

jillbee7 · Accepted Answer

A lot of this depends on what the drug is. I'll give you an example with a diuretic, which is a "water pill" and makes you pee a lot. You can give diuretics in 2 divided doses, but usually you would give them in the morning and early afternoon because if you took the second dose much later then you would need to get up a lot in the night to pee. These drugs have a relatively short effect, so taking them in the morning and early afternoon lets you get rid of a lot of excess fluid without having the effects during the night. Does that make sense?

Doses are divided for a few different reasons. They might be divided because they have a short effect and you want to try to sustain the effect over the whole day, hence dosing it more than once. You might also do it if larger dose give undesirable side effects, whereas smaller doses spread out over the day do not.

Do you mind me asking what drug this is that we're talking about? Because the maintenance dose says 20-60 mg in single or divided doses, I'm assuming that either would be effective but you may want to divide them to minimize side effects. Yes- 60 mg in divided doses means 20 mg three times a day, or 30 mg twice a day depending on what strength the pills come in. It would definitely be acceptable to do 20 mg at breakfast, 20 mg at lunch and 20 mg at supper, unless it is a drug that should  not be taken with food (but that's pretty rare).

Okay, your next question about elimination half life- first, let me explain what that means. It means that if you took one pill, your body would absorb as much as it was going to in 7 hours (peak concentration), and half of the dose would be inactivated or eliminated from your body within 12 hours (elimination half life). Then in another 12 hours another half of the remaining amount in your body would be eliminated, leaving you with a quarter of the original dose, and so on. This leads to a "build up" of the drug in your body, which is what we want when taking medication on a regular basis. A point will be reached which is called "steady state"- this is where the amount of drug in your body remains relatively constant as long as you take the same amount at the same frequency. Drugs that have longer elimination half lives stick around in your body longer, and so you don't have to take the drug as often to keep its levels constant in your body. That's the case with your example here- it's not all eliminated within 24 hours- in fact, 1/4 of it still remains. Over time this builds up, and so if you take the full dose once a day, that's okay.

As for your divide in 2 doses question, morning and dinner, morning and mid-afternoon, or even morning and evening could be acceptable. It all depends on convenience for you, possible effects of the drug, and if it should be taken with food or not. Talk to a pharmacist about this one- they will know!

I hope that wasn't too confusing and that it helped you out. Let me know if you have more questions or if I just confused you more!

Jill
(pharmacy student)

Multivinum · Answer

Thank you VERY much jillbee7 for taking the time and explaining in a very clear way! Very helpful! :)

Multivinum · Answer

Thank you VERY much cathleencat! :)

Multivinum · Answer

Thank you VERY much Creampuffy! :)

cathleencat · Answer

I agree with Creampuffy about how the doses work; however whoever prescribed this med should have indicated what the maintenance dose is: thus 20mg initally first morning and then 20mg twice a day morning and evening, freeing you from having to do all the work. Divided doses simply means splitting the total daily dose (eg. 40mg) into two doses which is often done to reduce gastric symptoms, for example, or because the med has a relatively short half life and you need to take it consistently to maintain a blood level. If this is a prescription drug, the dosages should be clearly indicated on the bottle strip. If not, you might want to check with a pharmacist at your local drugstore whom I'm sure can help you out. Good luck.

Creampuffy · Answer

Question no. 1

This will depend on what you are taking and for what purpose, for example ..a pain drug or an antibiotic


but as you have stated it means:


Initial dose:

                          the first day take 20 mg orally in the morning

From then on take:

                          20 mg once a day   or  

                                    

                          60 mg  once a day  or


                          20 mg  three times a day..........morning, lunch, supper


    On the second day it may be safer to take it three times a day in divided doses if you need the higher doseage.


Question no. 2

Absorption:

This means it takes 7 hours to reach it's peak performance

Elimination:

This means that by 12 hours it will be only working at half it's peak performance but still be in your blood doing it's thing. At 24 hours you will need another dose to keep the drug in your system.


You could divide your dose in two:   take it 12 hours apart. 

example: 7am and 7pm

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