Know when it is feasible for you to take that next dose
Most non-medical people who are on continuous medication which may be taken at their discretion, are told by their doctor or pharmacist that they may take a specific dosage only every so many hours. Some doctors will tell their patients, take one of this medication twice a day as needed? That's fine. You figure that means I should take it every twelve hours. If the medication is something like a pain reliever, you might find that the first four hours the pain was relieved, but then, after six hours it's back again. Do you have to wait another six hours before you can take the second? Actually, no. If you know the half life of the medication you're talking, then you can take it again, perhaps half a dose, after the quantity of the medication in your system has been reduced by half.
Most medications, especially pain medications, will be active and absorbed by your body over a set period of hours. It is the business of the pharmacist and doctor to know how long it takes for half of the quantity of the medicine you have taken has been used up by your system. Half the life of a quantity of medicine does not necessarily mean that 2 times the amount of time it took to reduce the original dose in half is the total amount of time it will take for all the medicine to be used up and absorbed. Let us say a quantity of medicine takes four hours before half of the original quantity is gone. This means precisely what it sounds like. In four hours, half of the medication will be absorbed and used by the body. This means that you now have only half the original strength in your body. You might notice your pain re-emerging after those first four hours, yet not at its peak when you first took the medicine. That's because only half the original strength is effective now.
Now, what you have in your body, the other half, will itself be absorbed and attain a new half life. Some medications in the second half of the full dosage will take as long to reach half its amount as did the first half. So, if you had a certain amount that reached half life after four hours, the remaining half will take another four hours for it to be reduced in half. That would leave a quarter of the original amount you took. If again, that remaining quarter takes four hours to reach half life, by the end of those four hours you'll have an eighth of the original amount you took. If the eighth takes four more hours for the remaining amount to be reduced by half, after those four, you'll have only a sixteenth of the original dose left. By then, the effect of the medication on your pain will be negligible. The point is clear here, Half of the life of a medication does not imply that all the medicine will be gone from your system. 2 time the original half life does not mean all the medicine has been used up. It may take many hours for it to completely disappear from your body.
We can see that, we could take a half of the original dose after the first half of the dosage had been used up and bring the strength of the medication back up to its original full strength. Of course, with any medication, you should always consult your physician or pharmacist when using this reasoning to regulate your medication regimen. Some medications may not so finely break out into halves as we've just described. A full stomach, other medications, your weight and other factors may also determine half life. Always be in doubt, and always check first before you take that next pill or a portion of it. Your pharmacist and doctor want your medication to work for you, not against.