What Is Drug Bioavailability in Simple Terms?

When you’re not feeling well, it’s common practice to take medicine. But do you ever think about drug bioavailability or what happens to the drug once you swallow it? How does it reach the part of your body that needs treatment?

Drug bioavailability refers to how much of the medicine you take successfully reaches the intended action site. Most of the drug gets metabolized in the liver and other organs before getting to the intended target. So, bioavailability is the fraction of the drug that reaches your systemic circulation in an active form.

We need to understand drug bioavailability for effective drug development and usage. Let’s start with the basics, from the simple definitions to the factors that affect it and some methods of improving bioavailability.

Simple Definitions to Understand Drug Bioavailability


This term refers to how the drug moves from the site of administration into your systemic circulation. When you take a pill, for instance, it has to be absorbed in your digestive tract before going into your bloodstream. 

Absorption is the initial stage of drug bioavailability. It can be affected by various factors including:

  • The route of administration.
  • Drug formulation.
  • Food presence in your stomach. 
  • Your gastrointestinal health.


After absorption, the drug is distributed through your bloodstream into various body parts. This term describes how a drug moves from the point of absorption into your bloodstream and to other body parts. 

Once absorbed, a drug is transported through the circulatory system to reach its intended site of action. Factors such as blood flow, tissue barriers, and the drug’s chemical properties can affect distribution. Other factors include:

  • The drug’s ability to cross cell membranes.
  • Blood flows to different organs or tissues.
  • Physicochemical properties of the drug.

For example, some drugs may have difficulty crossing the blood-brain barrier, while others may easily penetrate into fatty tissues.


Metabolism refers to the chemical modifications that occur in a drug as enzymes break it down. These changes can make the drug more active or inactive, decreasing its concentration in the body. 

The liver handles most human drug metabolism, although other organs like the kidneys and lungs may also be involved. Factors that affect drug metabolism include:

  • Genetics.
  • Age.
  • Diet.
  • Disease states.

Metabolism can also create drug metabolites, which may have different effects from the original drug. Some metabolites are inactive, while others may be toxic or even more potent than the parent drug.


The last stage of the pharmacokinetic process is excretion. It refers to eliminating drugs and their metabolites from your body through urine, feces, sweat, or breath. The body primarily relies on the kidneys for drug excretion, although it may also eliminate some drugs via bile, saliva, or breast milk.

So, what can affect the drug excretion rate?

  • Kidney function.
  • pH levels in urine and blood.
  • Blood flows to the kidneys.

Some drugs may also undergo enterohepatic recirculation, where they are reabsorbed into the bloodstream from the intestine and then eliminated again by the liver. This process can prolong a drug’s presence in the body.

What Is the Bioavailability of Drugs?

Bioavailability is a measure of how much of the drug you ingest reaches its target and is available to produce an effect. For example, if you orally ingest 100 ml of morphine, the liver is likely to metabolize over 97 ml, leaving only 3 ml to reach the target site and produce an effect.

However, doctors could inject you with 50 ml of morphine, and 100% of the drug will go directly into the bloodstream. However, your blood eventually goes through the liver and other organs. So, the bioavailability will gradually decrease as the drug gets metabolized and eliminated. 

So, bioavailability measures both the rate and extent to which a drug reaches its intended target. It’s a crucial factor in determining the dosage of a drug and how to administer it. 

Bioavailability affects drug effectiveness in the following ways:

  • Some drugs are only effective at a certain concentration in the body. If bioavailability is low, it may require a higher dose to reach that therapeutic level.
  • Low bioavailability is mainly because most of the drug gets metabolized by the liver and other organs, reducing its potency.
  • The route of administration can greatly affect bioavailability. For example, intravenous (IV) administration has 100% bioavailability, while oral administration is often lower due to first-pass metabolism in the liver.
  • Changes in kidney function can also affect bioavailability as they are responsible for filtering and eliminating drugs from the body. If kidney function is impaired, drug excretion takes longer and can increase the drug’s bioavailability.
  • Drug interactions also affect bioavailability. Certain medications or substances ingested together can impact how much of a drug is available to produce an effect.

How Does Food Increase the Bioavailability of Drugs?

Anyone who’s been sick knows that famous medical advice, “Only take this medicine after a heavy meal.” But why do doctors and pharmacists often recommend taking medication with food? 

Food increases drug bioavailability in several ways:

  • Taking medicine after food helps slow down the drug absorption rate by occupying the gut surface area where absorption takes place. It allows a longer time frame for the drug to reach its intended target.
  • Certain nutrients in food enhance drug absorption by increasing blood flow to the digestive tract or binding to the drug molecule, making it easier for the body to absorb them. 
  • Food can also protect drugs from stomach acid, which can break down some medications before they are absorbed. Taking medication with food provides this protective barrier, giving the drug enough time to reach its target in high potency.
  • In some cases, food can also delay gastric emptying, which entails moving food from the stomach to the small intestine. This delay can increase absorption time and, ultimately, the bioavailability.

Overall, taking medication with food can maximize the amount of drug absorbed by the body. However, it’s essential to note that this is not always the case. 

Some medications have decreased bioavailability when taken with food, so following your doctor’s instructions is crucial.

How to Improve the Bioavailability of Drugs

What is good bioavailability for a drug? Just enough of the drug should remain in circulation to produce the desired therapeutic effect. 

While food can increase drug bioavailability, other factors also come into play when optimizing drug absorption, including:

  • The timing: Taking medication at the same time every day is essential for maintaining consistent levels of the drug in your system.
  • The dosage: Pharmacists carefully calculate the recommended dosage to maximize effectiveness while minimizing side effects. Always stick to the prescribed dose unless advised otherwise by a healthcare professional.
  • The potential drug interactions: Did you know that some drugs react adversely when you take them together with particular drugs or food? You may not know what has adverse drug interactions with your dosage, so always consult your physician. 
  • Drug administration: How you ingest a drug also impacts its bioavailability. For instance, oral medications have lower bioavailability than injectable ones.
  • Product quality: Generic drugs may have varying levels of bioavailability compared to brand-name ones. It’s crucial to get your medication from a reputable source and check for any recalls or safety concerns.

Final Thoughts on Bioavailability of Drugs

Optimizing drug bioavailability is crucial for ensuring the effectiveness of your medication. However, a balance is necessary to avoid toxicity or adverse side effects. Always talk to your doctor to get accurate information and to determine if changes to your dosage are safe and effective. 

Did you also know that drug bioavailability is one of the main concerns among pharma companies when developing new medications? We consider it and how it affects pharmacological treatments when developing research chemicals.

Remember, only research can yield better and more tailored medical treatments in the future. So, how many new drugs are discovered each year, and what is bioavailability’s role in this process?