Correlation between the half-life & duration of effect of drugs?

[Anon54]

In a medical context, half-life may also describe the time it takes for the blood plasma concentration of a substance to halve ("plasma half-life") its steady-state.

There's obviously a correlation between the half-life of a drug & the duration of it's effects. The longer the half-life, the longer the effect. I can understand that, but I'm still confused as to how it exactly works.

For example; Xanax has a half-life of 6-12 hrs.
Does that mean if I take 4mg, approx. 9 hrs later I'll still be feeling like I've had 2mg of Xanax?
That sounds plausible.
But what about drugs like Valium which has a half-life of 20–100 hours (not including it's active metabolites)
Does that mean that if I take 80mg, approx. 60 hrs later I'll still be feeling like I've had 40mg of Valium? (not including it's active metabolites)

I also don't understand why Antidepressants can sometimes take up to 6 weeks to start working.
Lexapro for instance has a half-life of only 27–32 hours.
So in theory shouldn't it take Valium longer than Lexapro to build up to its peak in ya system because it has a longer half-life?

I'm changing my Antipsychotic med from Zyprexa which has a half 21–54 hours to Seroquel which only has a half-life of 6 hrs so hopefully I wont be effected as much during the day.
I'd like to know the differences in duration of effects between the 2, but it's obviously something u can't work out just from simply comparing there half-life's.

All this is probably not important. As long as I know the longer the half-life, the longer the duration. It's all I really need to know. I'm just curious.

 

[Pjkt2501]

Antidepressants work by slowly rearranging how the various chemicals in the brain are worked with. Excretion and reabsorption is effected, as well as the overall mechanism of action in some cases. They're vastly different from a bluntly-functioning painkiller, for instance.

Your hypothesis as to what a half-life means is correct. If the drug has a half-life of 4 hours, you'll still feel 2mg of a 4mg dose 4 hours after the drug has kicked in. 4 hours after that, 1mg is still present. The issue with this not always being the case (well, it is, but keep reading...) is that a person's tolerance level will usually make it so the latter parts of the time-frame are no longer effective in dealing with the issue that the medication is prescribed for.

If I get 2mg of Xanax to deal with a panic attack, and it initially works, but in several hours I'm still in a rough way, it'll no longer be as effective as when it kicked in, and a booster will be needed.

Blood-plasma concentrations of the drugs, in terms of time-duration, are also highly dose-dependent. The greater your dose, the longer it will stay in the bloodstream, and the more likely it will be for subsequent doses to compound on top of what you had previously taken (causing a greater and greater actual level of the drug in the bloodstream.

(Basically, if AP's kicked in right away, people would have SERIOUS problems, which is why they make slow changes.)

 

[Anon54]

So that's why u dont still feel high from valium the next day. because ya sort of get a tolerance to the effects?
that makes sense i guess
I was a little confused about the ADs though because i was told by the dr that when i was to try pristiq it would only take a week to start working because it has a much shorter half-life than most other ADs.

So do u know if the half-life of an AD as an influence on how long it takes to start working?

I just thought of something. It would seems that the drugs that arnt addictive & u dont get a tolerance to build up in ya body. that would explain why APs & ADs take a little time to fully start working

 

[Pjkt2501]

Some AD's are designed to work more quickly than others. It all depends on what exactly they're doing to the brain (and how comfortable the scientists making them are with having them make these changes at a particular rate).

I personally will still feel high from clonazepam or valium the next day if I take enough (I have to take at least 4-5mg of Clonazepam, which if I'm correct, is 80-100mg of Diazepam).

*By "high" I mean "notice something" - I don't get "high" in the traditional sense from benzos as I need them for my anxiety and they work quite well for what they're designed to do.

 

[Anon54]

did u read the edit i made with my last post?
explaining my theory of why non-addictive drugs build up in ya system. cuz ya dont get a tolerance to it like ya said
yeah I notice that to when I take like 100mg of diazepam. I wake up still feeling calm. unlike xanax

 

[Pjkt2501]

Xanax has a half-life of about 4 hours if I recall correctly.

AP's and AD's are very addictive / dependence forming. Think of how hellish it is to come off of either one cold-turkey...

 

[Anon54]

I had a look at the wikipedia benzos list & it says 6-12 hrs. not everything on that site is correct though
drs dont consider APs & ADs addictive because ya dont get a tolerance to them.
of course they're addictive though. its just the terminology the drs use.
they tend not to mention the part about w/d though.
ppl assume when the dr says its not addictive that there wont be any problems quitting em. of course that's not the case at all

 

[Pjkt2501]

Addiction = mental need
Dependence = physical need

 

[ms4104]

It's weird..heroin has a rael short half-life but my eyes are still pinned the next morning 12 hours after use.. and I don't start wtihdrawing for 24-36 hours

 

[Pjkt2501]

It's weird..heroin has a rael short half-life but my eyes are still pinned the next morning 12 hours after use.. and I don't start wtihdrawing for 24-36 hours

Potentially unknown crap cut into it. Nothing we can speculate on, however, as that would be pointless and a foolish guessing game.

 

[ms4104]

It's the heroin.. it's not some other drug. That is not an odd effect.. it's normal to have to wait 24-36 hours to dose your bupe and be in withdrawal from dope.

The heroin around here is very pure..and the cuts are inactive. They arnt cutting it with like methadone or some shit. Cuts are shit like vitamin b, etc..

 

[Pjkt2501]

Then what's your point exactly? I'm not sure how your comment aids the OP...

 

[ms4104]

My point is that it's a conundrum. The halflife of dope is so short yet it seems to linger in the system so long. I don't have an explanation... just that it's weird how half life relates to the duration with dope.

LOL my bad if I didnt aid the OP too good with my comment. Just like he is confused about the relationship between halfife and duration, I am..Just adding to the discussion man... is that ok?

 

[Anon54]

^^well i was told that the half-life doesnt mean when half the duration done. it's when half the drug has left the body. the other half will take longer than the first half to leave. the half life of h may be short but the "full life" (if that's the correct term) of the drug may be really long if u know what i mean

so ive figured that the reason ya not still high on valium a week later is because ya body gets tolerance to the effect.
drugs like APs & ADs u dont get a tolerance too so they can build up in ya body.

it makes sense to me now. unless anyone disagrees?

 

[Pjkt2501]

It is possible that the length of time a drug maintains a receptor affiliation is unrelated to the actual half-life of the drug, but I feel that this is something the pharmacist / producer of the drug sould ALSO be taking into consideration.

(Basically - I'm just as confused, but can kind of wrap my head around a possible reason or two).

Heroin induces euphoria. Think about how LSD leaves a glow for a day or two after its use, but once you're done tripping, it's gone. Again, I don't know for sure, but I think that because receptors don't just "click back" into their initial place / purpose you're getting the buzz for a longer period of time.