Drug Mechanism of Action | 9 Basic Types and Effects

This article brief about how the drugs of modern medicine work.

As per WHO, a drug is a physical, chemical or biological agent that helps to prevent, diagnose or cure a disease condition.

There are many medicine systems in the world like allopathy, Ayurveda, homeopathy, Unani, etc.

Of them, the drugs of modern medicine (allopathy) are widely used. They act by different mechanisms like

  1. Physical action
  2. Chemical action
  3. Physiological action
  4. By Receptors
  5. Substitutes
  6. Enzymatic action
  7. Killing foreign cells.
  8. Prophylactic vaccines
  9. Supplements

These drugs of modern medicine are highly prescribed due to the benefit of faster relief from the symptoms and suffering.

But on the negative side, some of these drugs have severe side effects limiting prolonged and widespread use.

These drugs are chemically diverse and even their mode of action is quite different.

For health-conscious people, it will be quite interesting to study the mechanism of action of drugs we regularly use.

Drug Mechanism of Action

Based on the mode of action, drugs can be categorized as the following types

Drugs acting by Physical methods

These are the drugs which do not react in the body. They do not alter the fluids or other biochemical substances. But they exert their effect just by being physically present at the location. The examples include

1) Bisacodyl: A drug of choice prescribed for peptic ulcers. This drug physically binds to the portion of the ulcer surface in the stomach. Thereby prevents the further attack of gastric acid on it. Due to a lack of further exposure to acid, the ulcer portion of the stomach gets healed faster.

2) Charcoal The activated charcoal is used as an antidote to poisons. Scientifically it is called activated as it in powdered form and made to readily adsorb matter.

This charcoal neither gets digested not absorbed in the gut. But it has a special tendency to adsorb substances. In the case of poisoning, it physically adsorbs poison in the gut and prevents it from being absorbed into the blood. Thus it minimizes the poison effect.

Thus the above drugs act by being physically present in the location of the problem.

Drugs acting by chemical reactions

Here, drugs act to produce relief by reacting chemically with some or other body fluids. The examples include

1. Antacids:  When you have acidity in the stomach, you are advised to drink a solution of sodium bicarbonate.

how drugs act-Drug Mechanism of Action

Acidity is caused due to the release of HCL in the stomach. HCl, as we know, is a strong acid. So when sodium bicarbonate a base (alkali) is consumed, it reacts with excess acid and neutralizes it to salt. Thus acidic pain is reduced. Other examples, in this case, are antacids tablets of aluminum hydroxide, magnesium hydroxide, etc.

2. Chelation or complexation:

Here the drug administered chemically bind with the poison molecule and inhibits its actions. Thereby the poison is unable to show its effects.

Example: Penicillamine is used to complex iron and copper in case of poisoning.

Citric juice: When there are kidney stones, physicians advise the consumption of a lot of citrus fruit juices like lemon, orange, etc. These juices are acidic in nature. The kidney stones are mostly made of Calcium salts. Calcium is an alkaline substance. So when acidic juice is taken in large quantities, it reacts with calcium (an alkali element) in the kidney stone and dissolves it. Also, any chances of further growth of the stone are inhibited by this citric juice.

Drugs acting by physiological modifications

Here, drugs produce some physiological effect and relief from symptoms of the disease.  This can be of three types.

1. Those producing the opposite effect

In case of severe diarrhea (loose motions without pain), the intestinal motility (movement) is very high. The movement is such that any food material in the intestine is pushed downwards, i.e., towards rectum for faster defecation. In this condition, doctors prescribe Morphine related drugs ( Ex; Loperamide).

Morphine has a special property in the gut. When taken orally, it reverses intestinal movement and instead of contents traveling downward, they travel up towards stomach and mouth. (Hence morphine consumption orally has symptoms like vomiting even undigested waste from the mouth). For severe diarrhea, weaker derivatives of morphine are given to slow down the intestinal run downward and stop loose motion.

2. Those producing related effects: Here, the body physiology is modified temporarily to bring down any adverse disorder.

Examples: In patients with high blood pressure, diuretics are given. Blood pressure is one that 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.

3. Those 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 region of application. Hence they are called rubefacients and also counter-irritants.

When there is a pain, if the balm or gel is applied at the point of pain, they produce 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.

Drugs acting 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 drugs of abuse function through this mechanism.

Drugs acting by substitution

Many vital drugs like anti-Parkinson’s,  anti-epileptics, etc. act by this mechanism.

Ex; In Parkinson’s disorder, there is a low concentration ratio of Dopamine with that of acetylcholine in the brain. Hence the Parkinson 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.

Enzymatic action

Few drugs like papain, urokinase act by their enzymatic action. They act as a catalyst to bring about some healthy change. Papain when taken after food, helps indigestion. On the other hand, urokinase helps to break clots formed in the body. Serratiopeptidase helps to reduce swelling.

By killing the foreign cells/ death mechanism

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

They kill the foreign cell or own body cells which are modified. These drugs are designed such that when consumed, they do not attack the 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 etc. These drugs lead to the death of cancer cells, bacteria, and viruses.

Prophylactic vaccines

Vaccines are the agents given to prevent susceptibility to infections. These vaccines are the derivative of the actual disease-causing agents. They alert the immune system in the body such that, future infections are quickly overcome without falling sick.


Drug Mechanism of Action

These are the agents which bring health by indirect means. For example, vitamin-B tablets help to increase the performance of the body. This is because of vitamin-B plays a role in metabolism.

This is a list of all the possible modes of drug actions. Most of the drugs fall into one or another category of the mechanism of action as mentioned above.

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