As per WHO, 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 Drug Action
Modern medicine drugs are widely used as they provide faster relief from the symptoms of a disease or disorder.
But on the negative side, they do not address the root cause of the disease and also have severe side effects limiting their prolonged and irrational use.
These drugs are chemically diverse, and their mode of action is quite different.
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 induce a change in the body’s chemistry or physiology.
But they exert their effect just by being physically present in the body at an intended location.
But such types of drugs are very few in number.
Examples of such drugs and their actions are.
- It is a drug of choice prescribed for 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 the harsh acid, the ulcer portion of the gut layer gets healed faster.
2) Activated Charcoal
- Activated charcoal is used as an antidote to poisons.
- Scientifically it is coal obtained from burnt wood. It 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 substances, it is administered for the treatment of oral poisoning.
- 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.
2. By Chemical Reaction
Here, drugs produce relief by bringing some chemical change in the body fluids.
The examples include
1. 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.
2. Chelation or complexation
- Here the drug administered into the body chemically binds with the poisons molecule and inhibits its actions.
- Due to this, the poison is prevented from causing further damage to the body.
- These complexed 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, it forms a complex iron and copper.
Thus it can be used for iron and copper poisoning treatment.
- 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 the kidney stones are mostly made of Calcium salts.
- Calcium being an alkaline substance, reacts with acidic fluids when taken in large quantities.
- Thereby due to the reactions, the calcium (an alkali element) stones in the kidney tend to dissolve.
- Also, any chances of further growth of the stone are inhibited by this citric juice.
By physiological modifications
Here, drugs produce some physiological effects and thereby give symptomatic relief from the disease. This can be of three types.
1. Those producing the opposite effect
- 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 towards 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 towards 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.
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 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 pain, if the balm or gel is applied at the point of pain, they produce 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.
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.
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.
- 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 indigestion.
- On the other hand, urokinase helps to break clots formed in the body. Serratiopeptidase helps to reduce swelling.
By killing the foreign cells
- 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.
These drugs lead to the death of cancer cells, bacteria, and viruses.
- Vaccines are the agents used 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.
- These are the agents who bring health by indirect means.
- For example, vitamin-B tablets help to increase the performance of the body.
- This is because vitamin B plays a role in metabolism.
- This is a list of all the possible modes of drug action. Most of the drugs fall into one or another category of the mechanism of action as mentioned above.