Pharmacokinetics facts for kids

This graph shows how much medicine is in your blood over time. The blue line goes up when you take the medicine, then slowly goes down as your body uses and removes it.

Pharmacokinetics is a part of pharmacology. It studies what your body does to a drug (medicine). It looks at how a medicine gets into your body, moves around, and then leaves. It helps us understand how much medicine you need to take to get the right amount in your body. It's connected to another area called pharmacodynamics, which studies how the medicine affects your body.

How Medicines Move in Your Body: ADME

When scientists create new medicines, they need to know exactly what happens to the medicine once someone takes it. There are four main steps that describe how a medicine moves through your body. We call these steps ADME:

• Absorption: How the medicine gets into your bloodstream.
• Distribution: How the medicine spreads throughout your body.
• Metabolism: How your body changes the medicine.
• Excretion: How your body gets rid of the medicine.

This ADME model helps make it easier to understand the many things that happen when you take a medicine.

Absorption: Getting the Medicine In

Absorption is about how a medicine moves from where you take it (like your mouth or skin) into your blood. The "bioavailability" of a medicine tells us how much of the medicine actually reaches your bloodstream.

What Affects How Medicine Is Absorbed?

Several things can change how well a medicine is absorbed:

• How you take it: Is it a pill, a shot, or a cream?
• Medicine's properties: How easily it dissolves or how acidic it is.
• Your body: Your sex, age, health, and how much blood flows to the area where you take the medicine.

Different Ways to Take Medicine

Medicines can be taken in many ways. Here are some common ones:

How Taking Medicine Affects Absorption

The way you take a medicine changes how quickly and how much of it gets into your blood:

How Fat Solubility and Acidity Affect Absorption

Once a medicine is in your body, it needs to enter many cells. Cells have a protective outer layer made of fat. Because of this, only medicines that can dissolve in fat (called lipophilic drugs) can easily get into cells.

Sometimes, a medicine might get stuck in fats that your body wants to get rid of. Then, it leaves your body without ever being absorbed. So, medicines need a good balance of being able to dissolve in fat and in water.

The acidity of a medicine also matters. Knowing if a medicine is acid or basic helps your body accept it and get the right effect.

• If a medicine is a weak acid, it absorbs better in the stomach (like aspirin).
• If it is a weak base, it absorbs better in the intestine (like codeine). If you swallow a weak base medicine, stomach acid might destroy it, so it won't work.

Taking Medicine by Injection

One way to avoid the stomach (where medicines can be destroyed) is to inject the medicine. This can be into the skin (intradermal), under the skin (subcutaneous), into a muscle (intramuscular), or directly into the bloodstream (intravenous).

• Intravenous (IV) injections put the medicine right into your bloodstream. This means 100% of the medicine enters your body, and it starts working very quickly (around 2 minutes). However, if injected too fast, there can be a lot of medicine in one spot, which can be a problem for some medicines like chemotherapy.
• Intramuscular injections into a muscle are the second fastest way. Medicines usually enter the blood in about 30 minutes. Muscles have a good blood supply, and exercise can even make absorption faster.
• Subcutaneous injections go just under the skin, between the muscle and the dermis. These are less painful and you can often give them to yourself. But absorption is much slower than IV or intramuscular injections.

Other ways to take medicine include applying it directly to the skin or a body surface. These are called topical medicines, like eye drops, creams for rashes, or lotions for eczema. Absorption is slow, but the medicine stays mostly in that one area, treating only where it's applied.

Nasal sprays and inhalers are other ways to get medicine into your body without it going through your stomach.

Distribution: Spreading Through the Body

Distribution is how a medicine moves around your body once it's in your blood. This movement can sometimes lower the amount of medicine in one place.

The "volume of distribution" is a way to describe how much space the medicine seems to spread into in your body. It's like a pretend volume of fluid that would hold all the medicine you took.

Once inside your body, medicine can move by simply soaking through membranes (diffusion) or by being carried in your blood. Your body has different "compartments" where medicine can go:

• Plasma (the liquid part of your blood)
• Fluid around your tissues
• Fluid inside your cells
• Fat

If a medicine has a low volume of distribution, it mostly stays in your blood. If it has a high volume of distribution, it spreads out into other parts of your body, like tissues.

What Affects How Medicine Spreads?

Many things affect how a medicine spreads in your body:

• Medicine's size and fat solubility: Smaller medicines can cross membranes more easily. Medicines that dissolve well in fat tend to build up in the brain and in fat.
• Blood flow: Medicines spread faster to areas with lots of blood, like your heart, lungs, and liver, compared to fat or bones.
• Binding to blood proteins: Many medicines attach strongly to proteins in your blood. Since these proteins stay in your blood vessels, the medicine also stays in the blood. Only the medicine that is "free" (not attached to proteins) can actually work and be removed from your body.
• Diseases: Some illnesses can change how medicines spread. For example, in meningitis, blood vessels in the brain become more open, letting some medicines cross into the brain.
• Special barriers: Your body has barriers, like the one protecting your brain, that stop medicines from entering areas where they might be harmful.

Metabolism: Changing the Medicine

Metabolism is how your body chemically changes a medicine. This usually happens in your liver, but it can also happen in your blood, lungs, stomach, intestines, kidneys, and skin. This process often turns the medicine into a "metabolite," which might be more or less active than the original medicine.

There are two main steps in metabolism:

• Phase 1: This step makes the medicine more reactive. Sometimes, this can make it more toxic or even more active.
• Phase 2: In this step, the changed medicine combines with another molecule. This usually creates an inactive compound that dissolves more easily in water. This makes it easier for your body to get rid of it.

Simply put, metabolism changes medicines that dissolve in fat into forms that dissolve in water, so they can be easily removed.

First Pass Metabolism

When you swallow a medicine, it goes from your small intestine to your liver through a special blood vessel. For many medicines, a large part of it is changed by the liver right away, before it even reaches the rest of your body. This is called "first pass metabolism."

For some medicines, so much is changed by first pass metabolism that only a small amount (like 10%) actually gets into your bloodstream. Sometimes, this metabolism even happens in your gut before the medicine reaches the liver.

Excretion: Getting Rid of the Medicine

Excretion is the process where your body removes a medicine or its changed form (metabolite). This usually happens through your kidneys (in urine), but it can also happen through your liver (in poop), skin (sweat), and lungs (when you breathe out).

What Affects How Medicine Is Excreted?

Many of the changed medicines from the liver dissolve in water. These then travel to the kidneys and are removed in your urine.

Medicines or metabolites that dissolve freely in water can also be removed in sweat or when you breathe out. If a medicine or metabolite doesn't dissolve well in water, it's usually sent into your gut through the bile duct and then leaves your body in your poop.

Sometimes, a medicine that's excreted into the gut can be reabsorbed back into the body and sent back to the liver. This is called the enterohepatic shunt. It means the medicine stays in your body longer, which can make its effects last longer if the changed medicine is still active.

Some chemicals, like lead, can build up in your body's tissues instead of being easily removed.

How Much Medicine Is in Your Blood?

The amount of medicine in your blood depends on how well it's absorbed, distributed, metabolized, and excreted. If you take the same medicine by IV injection versus swallowing it, the amount in your blood will be different. For example, only about half of a swallowed dose might enter your blood.

Most of the time, medicines are taken in multiple doses, not just one. When you take medicine regularly, the amount in your blood will go up and down, but it will eventually reach a "steady state." This is when the amount of medicine entering your body equals the amount leaving it. At this point, you usually get the best effect from the medicine.

Images for kids

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  Different types of pills, which will act differently in your body after you take them.

See also

In Spanish: Farmacocinética para niños