Why Does My Pharmacist Give Me Instructions? All About Drug Absorption


 By Super-Pharm

If you take prescription drugs, you may have received directions from your pharmacist or doctor about how to take them: morning, afternoon, or evening; before, during, or after a meal… and you may be wondering why these instructions matter. In this article, we will explain some of the mechanisms that affect the way a drug is absorbed.

Duration of Action

Duration of action is the length of time that the effects of a drug can be observed in our bodies. This length of time is influenced by a variety of factors, but as a general rule, our bodies start to break down the drug as soon as it is digested, and a drug that breaks down into chemicals that are still active will last longer than one that is only effective in its original chemical composition.


The coating is one factor that can influence a drug’s duration of action Not all drugs have coatings, and the ones that do may have different types that are meant to serve different purposes. The simplest type of coating is merely to protect the drug from oxidization or moisture and maintain its freshness and stability.

Some drugs are coated with a compound that protects the esophagus and stomach from coming into contact with a drug that may irritate these organs and cause nausea and stomachaches or, with prolonged use, local inflammation or ulcers.

Another type of coating protects a drug from acidity in the stomach, which may cause it to break down or become ineffective before it reaches the intestine. These coatings can withstand the high-acidity (low pH) environment, but once they reach the duodenum or small intestine, which are low-acidity (high pH), they break down and allow the drug to be absorbed. This type of coating is called an “enteric coating.” Drugs with an enteric coating should be taken on an empty stomach, because the acidity level decreases while the stomach digests food, and in a higher-pH environment, the drug may start to break down before it has reached the duodenum. That can change the way the drug is absorbed and consequently, its effects.


Bioavailability is the proportion of a drug that is absorbed into the body and reaches the bloodstream. For example, the active ingredient in a 500mg Acamol pill is paracetamol (also known as acetaminophen), and if we can only absorb 400mg of the available paracetamol, its bioavailability is 80%.

Bioavailability differs from person to person based on their metabolism, age, and other factors, so it’s normally expressed as a range—for example, 70-90%.


Every drug, whether taken by mouth, injection, suppository, etc., should have a controlled effect on the target organs. While the drug is at work, our body continues clearing waste out of our systems.

Waste elimination occurs through a few different systems and organs, and the liver—which breaks down chemical compounds in the body—is central to this process. The drug will be broken down by the liver and will eventually be expelled from the body through one of the elimination systems, such as the urinary system, the digestive system, the sweat glands, etc.

Normally, the concentration of a drug in the blood system and in the target organ rises shortly after taking the drug, and then gradually lowers. In effect, each drug engages in a sort of “race against time” to have the maximum effect before it is expelled from the body.


The “half-life” of a drug defines the amount of time it takes for the concentration of a certain drug in the bloodstream to go down by half. The higher the half-life, the longer the drug stays in our system in its initial composition, and this usually means its effects will be longer-lasting.

The half-life of a drug is influenced by many factors, such as: how widely the drug is dispersed (it it’s absorbed by the fatty tissues, or just the cells, for example), its molecular composition, and how easily the body is able to eliminate it.

Maximum Serum Blood Concentration

This defines the maximum amount of the drug that can be concentrated in the blood. If the drug is taken once, it will reach its maximum serum blood concentration fairly soon after it is taken and will gradually decrease until it’s been completely eliminated from our bodies. If the drug is taken regularly, however, its blood concentration will rise, since it won’t have a chance to completely leave our systems, and another dose will add to the amount already found in the blood. In general, if you take a drug regularly, after four “half-lives,” the drug will reach saturation—that is, a situation where the concentration can no longer rise even if you take more doses, and will stay at the same level from that point.

Maximum serum blood concentration has therapeutic implications. For example, when treating high cholesterol, we want the maximum serum blood concentration to coincide with the time the liver is producing cholesterol. This occurs at night, during sleep, and therefore, doctors usually recommend taking cholesterol-reducing drugs right before going to sleep.

Food & Drink

Some drugs are absorbed better when they are taken with food, and some on an empty stomach. There are a few explanations for this:

  • An active ingredient that is fatty will be absorbed better alongside other fats
  • An active ingredient that is absorbed better in a higher-pH environment is better taken after a meal, since the stomach will be less acidic
  • A drug without an enteric coating may irritate the stomach lining, and is therefore best taken along with food, which can prevent it from having too much contact with the stomach lining

However, in most cases, taking a drug before or after a meal doesn’t have that much significance. Even if the food increases or decreases its absorption, these changes are minimal and the drug will likely still be effective.

Dosage Frequency

All these parameters can explain how dosage frequency works. There are medications you can take once per day that will last all day (such as the allergy medicine Lorastin), and medications, such as Sinufed, that need to be taken thrice per day.

The longer the drug stays in your system, the fewer times per day you need to take it, and vice versa. The duration of the drug’s effect depends, as we have seen, on many factors, such as bioavailability, half-life, active metabolites, and more.

In the case of antibiotics, it´s very important for the drug to maintain a steady concentration level in the blood. The bacteria that cause infection don’t sleep at night; they are constantly multiplying. Therefore, there must be enough of the antibiotic in the blood to kill them off at any given moment.

Because of this, the general practice is to divide the day into equal parts. Some antibiotics can be taken once a day (such as Azenil), since the medication is absorbed into the tissues, and is slowly and steadily released back into the bloodstream. It stays active in our systems even when we’re no longer taking it.

It’s also important to take the full course of antibiotics as prescribed by your doctor. Stopping in the middle, when some of the bacteria may not have been completely destroyed, opens a window for the bacteria to build resistance against the antibiotics, so they will no longer be affected by it—which will require continued treatment with a stronger antibiotic.

Taking Multiple Medications Per Day

It’s important to take medications at the prescribed times. Some medications should be taken in the morning, while others should be taken at night. Your doctor or pharmacist will give you directions. Generally, you can take multiple medications together, provided there is no reason to separate them.

You may be directed otherwise, for example, if one medication must be taken with a meal while another must be taken on an empty stomach, or if there is some kind of interaction between the drugs that may affect their function or absorption. Make sure you are familiar with the doctor’s instructions and be sure to consult a professional if you have any doubts or questions.


Super-Pharm is the largest and most popular pharmacy chain in Israel. Super-Pharm Big Fashion is located in the Big Fashion Mall, Yigal Alon 3, Beit Shemesh.

Adi Levy, manager of the branch, can be reached directly at

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