![]() The morphology consists of fine pink macules or papules hence, it is sometimes called a maculopapular eruption.It can look similar to a viral exanthem but is usually pruritic, whereas similar viral rashes are not particularly itchy.A morbilliform rash usually starts within 7–14 days of starting a new antibiotic, and lasts for 5–10 days. It may occur more quickly on re-exposure to the same drug.Many antibiotics cause morbilliform eruptions.Some morbilliform eruptions do not recur when the patient is re-exposed to the causative drug. Morbilliform eruption is the most common type of antibiotic rash.It usually takes 7–10 days to become allergic to a drug, so if a reaction is rapid, it is either non-immunological, or it is due to a previous encounter with the same drug or a chemically similar substance. Rashes due to antibiotics are most often morbilliform ( exanthematous) or urticarial. How are cutaneous drug reactions classified? Severe ADRs to some antibiotics, such as Stevens-Johnson syndrome/ toxic epidermal necrolysis (SJS/TEN) due to sulphonamides, are caused by complex immunological mechanisms. Most severe cutaneous adverse reactions are Type IV reactions. They are caused by immune T-cell responses and release of cytokines. Type IV hypersensitivity reactions or cell-mediated delayed responses to drugs develop over several hours.Type III reactions can occur 1–3 weeks after exposure. Type III hypersensitivity reaction results from drug and antibody immune complexities deposited in tissues (eg, cutaneous vasculitis, deposition and subsequent complement activation in the blood vessels of the skin leads to palpable purpura).Type II sensitivity reaction is less common and is due to the development of specific antibodies to the drug, resulting in stimulation or inhibition of immune pathways.This type of reaction results in anaphylaxis, angioedema, and drug-induced urticaria due to rapid vasodilation and increased vascular permeability. Type I hypersensitivity reaction is an immediate hypersensitivity reaction to a drug and occurs within minutes to hours, due to pre-formed antigen-specific immunoglobulin (Ig) E antibodies and mast-cell degranulation.These immunological ADRs make up approximately 20% of ADRs. There are four main types of immunologically mediated hypersensitivity or allergic responses to drugs. Pellagra can result from long-term use of the anti-tuberculous agents isoniazid and ethionamide due to inhibition of the tryptophan-niacin pathway. ![]() Ototoxicity (damage to the inner ear) due to the build-up of aminoglycosides in renal failure.Infection with other organisms such as yeasts (eg, oral candidiasis or vulvovaginal candidiasis due to broad-spectrum antibiotics, or Malassezia folliculitis or Gram-negative bacterial folliculitis due to long-term treatment with a tetracycline).Non-immunological ADRs caused by antibiotics include: They can occur on a single occasion or on every occasion the drug is prescribed.The ADRs can also be unpredictable and idiosyncratic.Dose-related ADRs may be due to underlying renal or hepatic disease.The reactions are often predictable pharmacological side effects. ![]()
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