Does taking half of a pill (say pain pill) provide you with half the relief of a full pill or half the duration of a full pill or both?
Depends on the medication, and the formulation of that particular tablet. The pharmacokinetics of a medication can change with so many variables that it is hard to generalize… But this is the dope, so I’ll give it a try 
If you take a normal immediate release tablet, and cut the tablet in half, and the metabolism of the drug isn’t saturable, and follows 1st-order kinetics, then you will receive half the dose, and it will be at sub-therapeutic levels in half the time.
I’m on Warfarin, and the pill and my dosage is designed so that I take 2 1/2 pills four days and 3 pills the other three days. So, for that example, 1/2 pill is one half the dosage. The pills are also scored for easy breaking into two.
IR not a doctor etc. But often no, the effects of some drugs can be very non-linear, and sometimes increasing the dose diminishes the effect. Some drugs need a minimum dose to be effective at all. It depends, if in doubt ask a pharmacist.
Here’s a piece on the biphasic effects of alcohol.
I hope you won’t mind if I attempt a translation of this for the Teeming Masses:
First off, if it is a time-release tablet, cutting it exposes the insides, and kills the time-release mechanism, so all bets are off. Next, you also have to know how the pill was supposed to dissolve; some drugs are useful even in the throat, but others could lose all their strength there and be useless by the time it gets where it’s supposed to be. Finally, some drugs need a certain minimum dose for any effectiveness at all; you think you’ll get half the power but the truth is that your body might not even notice that you took any medicine at all.
If the drug is scored, then it was designed to be split and has been proven to split reliably to give half (or quarter) of the dose within a certain range (+/- 2%, I’d guess). If the drug is not scored or otherwise intended to be split, then there isn’t a soul on the planet who can tell you what you’ll get out of it.
Drug manufacturers do not test drugs to see how they perform when used in ways that they are not intended to be used (trust me, I used to do the testing). This includes splitting, crushing or administering the drug in a manner other than indicated. You might get half the dose, you might not, since it might not split evenly. You might expose the drug molecule to an environment that it cannot handle without the coating on the tablet, such as moisture, stomach acid or even the pH of the upper instead of the lower GI tract. You might cause the drug to be less available for you body to absorb, or maybe cause it to be absorbed faster than intended, thereby either undermedicating or overmedicating and/or increasing the risk of worsened symptoms or side effects.
Take medicine as directed on the label, and I’d be more likely to trust a pharmacist than a doctor when it comes to instructions regarding scoring/cutting/crushing/alternate administration if you choose not to believe the 10+years of development and testing that went into writing the label in the first place.
Warfarin is special in a couple ways. I take it too and my doctor explained it carefully. First off, it has a log dwell time, so that so long as you take the right average dose, the details of your regimen are not important. At least the coumadin brand comes in sizes of 1, 2, 4, and 5 mg, I think. People differ enormously in their proper dosage and there is not much room between an ineffective and a dangerous dose. So I am monitored every 6 weeks and sometimes my dosage does change. Normally I alternate between 5 mg and 7.5 mg. But I got an antibiotic yesterday and the doctor told me to halve my dosage for the duration (10 days) because an antibiotic can increase the activity of the drug. He didn’t tell me exactly how to halve it, but I worked out that I should take 5, 2.5, 2.5, 2.5 and start over. That gives me 12.5 mg every four days, where I normally get 25. My point here is the up and down don’t matter since it is only the average dose that counts. But that’s for warfarin. There is no reason to think that this will be true for any random drug; I would guess it is unlikely, in fact.
Exactly. If drugs all behaved the same, we wouldn’t have to test them so much! The behaviour of one drug - down to whether or not you can split it - cannot predict the behaviour of another drug, even if the molecules are related. Even changing excipients (non-active ingredients) can change a lot of the drug’s behaviour; sometimes this is desirable such as in making new extended release formulations, but sometimes testing shows it’s a bad idea due to interactions or poor or excessive drug release, etc.
Even pharmacists seem confused on this. I asked several different ones about Ziac (high blood pressure) and splitting. The pharmacists at Walmart and CVS said, “Sure, split it.” The ones at Osco and Walgreens to me no you shouldn’t.
Go figure huh?
As mnemosyne said, it all depends on the studies made on the absorption of the individual tablets. There are times when no studies have been made on a tablet, and no one actually knows what happens. That is when there is a “best guess” made. Now, as a pharmacy student who has time to actually research (a little at least) these things, there are no studies on Bisoprolol/HCTZ combination tablets as far as the effects on splitting it, however, Bisoprolol individually and HCTZ individually are ok to split. So, that implies that there are no negative effects with the combination Bisoprolol/HCTZ tablet encountering the low pH of the GI tract. With my educated opinion there would be no problems with splitting Ziac tablets…
Though, as mnemosyne said, the problem is no one actually knows for sure since it hasn’t been studied. Most retail pharmacists don’t have the time to do the research required due to the requirements made on them by the chains they work for. Unfortunately there is no one resource to tell if a tablet is ok to split, so in most cases it takes an educated opinion to say one way or the other. To make an educated opinion requires time to research the information, which most retail pharmacists don’t have, especially if you go during the rush times requesting information. If there is an obvious No, or an obvious Yes answer, they will give it without hesitation, but the maybe’s depend on the pharmacist… With Bisoprolol/HCTZ, there are no studies on the combination, I had to look at the individual components to come with an answer.
The thing is, the formulation - the recipe - for the tablet itself might (and indeed is very likely to be) different for a combo drug than for each of them individually. If one medicine is intended for fast-release and the other is intended for slow-action, then it might be a dual-layer tablet or be mixed in such a way that an excipient hinders the release of Drug A while Drug B gets released immediately, etc. Splitting such a tablet risks affecting the rates of release (dissolution profile), nevermind the rates of absorption by the patient. Very small changes in recipe can have rather surprising results in changing the dissolution/release profile of a drug. I vaguely recall on in which the amount of lactate was varied from about 28 to 33% (I forget that the balancing ingredient(s) were), and the drug release to 95% varied between 5 and 20 minutes!
Naturally, something that’s safe individually doesn’t become immediately toxic when combined, and that’s where a pharmacist’s education and experience kicks in to help a patient deal with the medicine they have to take, but at the end of the day it was never tested and no one knows. There is always a risk that cannot be removed when you take something in a manner other than indicated, in addition to the existing risks of taking medication.