Drug Mechanism of Action | 9 Basic Types and Effects

A drug is a physical, chemical, or biological agent that helps to prevent, treat, cure, or diagnose a disease condition.

There are many drugs among the different medical systems, like Allopathy, Ayurveda, Homeopathy, Unani, etc.

Of them, modern medicine (allopathy) drugs are widely used. 

Mechanism of action of drugs

The drugs are of different types and have diverse modes of action.

For simplicity, the drugs can be categorized into the following types based on the mode of action.

1. Physically acting drugs

These are the drugs that do not undergo any change or induce any anatomical or physiological change in the body.

But they show their effect by being physically present at an intended location.

These types of drugs can be understood by means of a few examples like

Activated Charcoal

• Activated charcoal is used as an antidote to poisons.

• Scientifically, it is the coal obtained from burnt wood which is then powdered and treated with steam to enhance its adsorption power many folds.

• This charcoal neither gets digested nor absorbed in the gut but is expelled from the rectum.

• However, due to its tendency to adsorb (attach or bind) substances, it is administered to treat oral poisoning.

• Here, it physically adsorbs poison in the gut and prevents it from being absorbed into the blood.

• Thus, it minimizes the poison effect.

Bisacodyl

• It is a drug prescribed for treating peptic ulcer disease in the gut.
• This drug physically binds to the surface of the ulcer in the stomach and therefore avoids any further exposure to the gastric acid released.
• Thus, by preventing further exposure to harsh acids, the ulcer portion of the gut layer heals faster.

2. Acting by Chemical Reaction

Here, drugs produce relief by bringing some chemical change in the body fluids.

The examples include

Antacids for gastric acidity

• When you have acidity in the stomach, you are advised to drink a solution of baking soda (sodium bicarbonate).

• Acidity is caused due to the release of Hydrochloric acid (HCl) in the stomach.

• HCl, as we know, is a strong acid, and it gets neutralized in the presence of an alkali or base.

• So when sodium bicarbonate, a base (alkali), is consumed, it reacts with excess acid and neutralizes it to salt.

• Thus acidic pain is reduced instantly.

Other examples, in this case, are antacid tablets of aluminum hydroxide, magnesium hydroxide, etc.

Chelates or complex forming agents

• Here the drug administered into the body chemically binds with the poison molecule and inhibits its actions.

• Due to this, the poison is prevented from causing further damage to the body.

• These complexes are water-soluble and are removed through the urine.

• Thus the poison is also safely removed from the body.

Example: Penicillamine, when administered in heavy metal poisoning, forms a complex of iron and copper.

Thus it can be used for iron and copper poisoning treatment.

Citric juice

• When there are kidney stones, physicians advise consuming a lot of citrus fruit juices like lemon, orange, etc.

• These juices are acidic in nature, and kidney stones are mostly made of Calcium salts.

• Calcium, an alkaline substance, reacts with acidic fluids when taken in large quantities.

• Due to the reactions, the kidney’s calcium (an alkali element) stones tend to dissolve.

• Also, any chances of further growth of the stone are inhibited by this citric juice.

3. By physiological modifications

Here, drugs produce physiological changes that can give symptomatic relief. This can be like

Producing opposite effects

• In severe diarrhea (loose motions without pain), intestinal motility (movement) is very high.

• The movement is such that any food material in the intestine is pushed toward the rectum for faster defecation.

• In this condition, doctors prescribe Morphine related drugs ( Ex, Loperamide).

Morphine has a special property on the gut. It reverses intestinal movement, and instead of contents traveling downward, they travel up toward the stomach and mouth. (Hence morphine consumption orally has symptoms like vomiting and even undigested waste from the mouth).

For severe diarrhea, weaker derivatives of morphine are given to slow down the intestinal movement downward and stop loose motion.

Producing related effects:

Here, the body’s physiology is modified temporarily to reduce any adverse disorder.

Examples: In patients with high blood pressure, diuretics are given. Blood pressure exerts pressure on the blood vessels.

• When blood volume increases, the pressure also increases.

• So the drugs are given to decrease the water in the body by enhancing the urine output.

• Due to this, blood volume decreases, thereby reducing blood pressure.

By producing an unrelated effect

• Here, the symptoms are mitigated by producing unrelated changes in physiology.

• Drugs containing menthol, capsaicin, and other pain-relieving balms. These gels show this unrelated mechanism of drug action.

• The volatile oils present in these drugs produce irritation and redness in the application region. Hence they are called rubefacients and also counter-irritants.

• When there is pain, if the balm or gel is applied at the point of pain, it produces an irritable and burning sensation at the place of application.

• This leads to high blood circulation at the point, causing redness and swelling of the region. 

Hence the actual pain is nullified or forgotten due to a new irritating or burning feeling.

4. Through receptors (pharmacology based)

• This is a common method by which most drugs used in important disorders function.

• Receptors are situated at the cellular surface or rarely inside.

• When drugs bind to them, they bring changes at the cellular level and help relieve symptoms.

Ex; Most drugs used in depression, schizophrenia, anxiety, and abuse function through this mechanism.

5. By substitution

Vital drugs like those for hypothyroidism, anti-Parkinson’s,  anti-epileptics, etc., act by this mechanism.

• In hypothyroidism, the thyroid hormone thyroxine secretion is low.

• To combat the issue, levothyroxine made externally is administered.

• Similarly, in Parkinson’s disorder, there is a low concentration ratio of Dopamine with acetylcholine in the brain.

• Hence Parkinson’s symptoms are present. To minimize them, Levodopa is given.

• This is similar to dopamine in chemistry. In the brain, it breaks down into dopamine and enhances concentration.

• Thus the imbalance in the ratio of dopamine and acetylcholine is minimized.

• Due to this, Parkinson’s symptoms subside.

6. By Enzymatic action

• Few drugs like papain and urokinase act by their enzymatic action.

• They act as a catalyst to bring about some healthy change.

• Papain, when taken after food, helps with indigestion.

• On the other hand, urokinase helps to break clots formed in the body. Serratiopeptidase helps to reduce swelling.

7. By Cytotoxic action (killing the foreign cells)

• This is the mechanism of action for anticancer, antiviral, and antibiotic drugs.

• They kill the foreign cell or their own body cells, which are modified.

• These drugs are designed so that when consumed, they do not attack normal body cells.

• But they attack invading microbes.

• They do so by inhibiting the vital metabolite of cell physiology or destroying vital structures like the cell membrane.

These drugs lead to the death of cancer cells, bacteria, and viruses.

8. Prophylaxis (vaccination to prevent infections in the future)

• Vaccines enhance the immunity or build immunity against a specific infection.

• These vaccines are the derivatives of the actual disease-causing agents.

• They alert the immune system so that the body resists future infections.

9. As Supplements

Here, the medicine helps in producing extra or added effects.

These are the agents that promote health by indirect means by aiding the existing process.

For example, vitamin-B tablets help to increase the performance of the body as they are part of many enzymatic reactions.

Thus, they can enhance metabolism when taken in addition to food as supplements.

Conclusion

This is the list of all the possible modes of drug action. However, few drugs tend to follow two or more mechanisms to relieve disease.

References:

• Drug Action
• Principles of Drug Action
• Classification of drug actions