Adverse Drug Reactions

Adverse drug reactions are reactions that occur apart from the normal drug action. Adverse drug reactions are unwanted reactions and are often seen if higher or inappropriate doses of medicines are taken. They are also known as side reactions. There are two types of adverse drug reactions:

  1. Occur with the normal pharmacological profile of the drug: Type A reactions: Predictable effect. In this case the reason is known and the conditions can be reversed by reducing the dose or switching over to another medicine.
  2. Not related to the profile of the drug or Type B reactions: Non – predictable effect: is mainly due to genetic differences in the metabolism of the drug, immunoligically mediated effects and the main reason is that the outcome can be fatal since the mechanism for the adverse reaction is not known.

Common Adverse Effects of Drugs

A picture of a common adverse drug reaction - the skin rash (reference 3)

A picture of a common adverse drug reaction - the skin rash (reference 3)

The following are some of the most commonly seen adverse drug reactions. They also have their causes listed under them.

NOTE – The causes highlighted are generalizations. They are in no way a complete list. Every drug and medicine will have its own adverse drug reactions. The causes stated are just generalized for the class of medication.

Respiratory Depression

Respiratory depression is a pattern of regular respirations with a rate of less than 12 breaths a minute. The respiratory centers regulate the breathing in coordination with the medulla and pons. When there is a reduced cerebral perfusion to activate the neurons of the respiratory center or when there is a change in the levels of arterial carbon dioxide and they trigger the chemoreceptors, or when there is a reduced effectiveness in changing the levels of carbon dioxide by the respiratory neurone there can be respiratory depression.

Cause: Mainly caused by CNS depressants.

Anaphylactic Shock

Anaphylactic shock is an acute dramatic reaction characterized by respiratory depression, angio – oedema, cardiovascular collapse, vomiting and urticaria. It is an exaggerated allergic reaction.

Cause: Mainly when there is an allergic reaction to the drug for e.g. penicillin.

Dizziness or Giddiness

Dizziness is a sensation of imbalance, associated with weakness, confusion , blurred or double vision. These episodes are usually short . it is mainly due to irregular blood flow to the brain and spinal cord.

Cause: CNS depressants, narcotics etc


Constipation involves infrequent and difficult bowel movements. It is the ANS which is responsible for controlling the bowel movements. It may lead to lack of appetite and abdominal discomfort.

Cause: Narcotics, analgesics, antacids, too much laxative.


Blood pressure relates to the force exerted on the blood vessels and is affected by the cardiac output, peripheral vascular resistance and blood volume. Raised blood pressure is referred to as hypertension and in this case the arterial walls thicken , become less elastic and resistant to blood flow.

Cause: sympathomimetics, corticosteroids, oral contraceptives, MAOI, CNS stimulants


Hypotension is decreased blood pressure to oxygenate the body tissues. Can occur due to vasodilation, dehydration and severe bleeding.

Cause: calcium channel blockers, diuretics, antihypertensives, general anesthesia.

Oral Candidiasis

Oral candidiasis is a mild superficial fungal infection caused by the candida species in the mouth. These bacteria are found in the normal flora of the mouth and when the drug alters the balance of this flora it leads to infection and can result in blue – white patches on the tongue, mouth and pharynx.

Cause : Generally seen in antibiotics


Type of skin eruption- which is generally caused as a result of an allergic reaction. This is a hypersensitivity to the medicine.

Cause: Generally seen in antibiotics

Dry Mouth

Decreased salivation, or dry mouth, involves the decreased production of saliva resulting from mouth breathing.

Cause: antimuscarinics, narcotics and excessive irradiation of the mouth or face.


Nausea involves profound aversion of food or an impeding desire to vomit. The medication tends to produce this by stimulating the vomiting centers in the medulla oblongata or by irritating the GI tract.

Cause: narcotics, analgesics

Drowsiness and Sedation

Drowsiness or sedation is a decreased level of consciousness follows after the use of medication that depresses the CNS.

Cause: This is observed a lot in case of anti-histamine drugs and anti allergy medication.

Fever or Pyrexia

Fever or pyrexia arise from any medical condition that can affect the body system. It is generally characterized by a rise in the normal body temperature and affects largely the homeostasis of the body.

Cause: hypersensitive reactions, chemotherapy, drugs that impair sweating.


Photophobia is an abnormal sensitivity to light results from ocular dilation and reduction in aqueous humour drainage.

Cause: mydriatics, ophthalmic viral drugs


Stomatitis is characterized by recurrent, painful ulceration of the oral mucosa. Can be a cause of an allergic reaction.

Cause: Cytotoxic drugs, radiation therapy.


Diarrhoea is an increase in the frequency and fluidity of the bowel movement. It is also known as loose motions or loose stool.

Cause: antibiotics, antacids, laxative abuse.

Angogenital Candidiasis

Angogenital candidiasis is a mild fungal infection caused by the candida species in the vaginal, anal or penile areas.

Cause: Oral contraceptives, other oestrogen containing preparations.


Vomiting is the expulsion of the gastric contents by the mouth as a result of coordinated contraction of abdominal muscles and reverse oesophagal peristalsis.


Blistering is a small thin walled raised vesicle containing clear, serous, purulent or bloody fluid. Can result due to allergy reaction.

Causes: Antibiotics, cytotoxic drugs.


Photosensitivity is an increased reactivity to sunlight. It is different from photophobia. In this case the body shows reactions physically – as in rashes etc, upon exposure to sunlight.

Causes: anticancer drugs, phenothiazine

Postural Hypotension

Postural hypotension is an abnormally low bp that occurs when an individual takes a standing position. It is often associated with medication that block the alpha – adenoreceptors.

Cause: sympatholytics, phenothiazines antihypertensives.


  1. Goodman & Gilman’s The Pharmacological Basis of Therapeutics by. Joel Griffith Hardman, Lee E. Limbird, Alfred G. Gilman. 10th Ed.
  2. Rang & Dale’s Pharmacology by. Humphrey Rang, Maureen Dale, James Ritter, Rod Flower. 6th Ed.
  3. The Skin Care Journal (website) – (accessed on April 02, 2011).

Related Posts

About The Author

Add Comment

I accept the Privacy Policy

This site uses Akismet to reduce spam. Learn how your comment data is processed.