Irritants and Counter-Irritants

| Home | | Pharmacology |

Chapter: Essential pharmacology : Drugs Acting On Skin And Mucous Membranes

Irritants stimulate sensory nerve endings and induce inflammation at the site of application. Depending on their nature, concentration and sensitiveness of the site, they produce cooling sensation or warmth, pricking and tingling, hyperaesthesia or numbness and local vasodilatation.


IRRITANTS AND COUNTER-IRRITANTS

 

Irritants stimulate sensory nerve endings and induce inflammation at the site of application. Depending on their nature, concentration and sensitiveness of the site, they produce cooling sensation or warmth, pricking and tingling, hyperaesthesia or numbness and local vasodilatation. Irritants which cause local hyperemia with little sensory component are called Rubefacients. Stronger irritants which in addition increase capillary permeability and cause collection of fluid under the epidermis (forming raised vesicles) are termed Vesicants. Certain irritants also produce a remote effect which tends to relieve pain and inflammation in deeper organs—called Counterirritants.

 

Mechanism Of Counter-irritation

 

Cutaneous sensations are precisely localized. Deeper sensations from muscles, joints and viscera are perceived more diffusely. A spinal segment, receiving afferent impulses from the surface as well as from deeper organs, modulates them—preferentially conducting the former to the higher centers. When a counterirritant is applied to the area of skin supplied by nerves form the same segment as the deeper organ form which pain impulses are coming, the cutaneous impulses obscure the deeper sensation.  Irritation of afferent nerve endings produces arteriolar dilatation in the adjoining areas of skin by axon reflex (which mediates flare in triple response). Through segmental association of afferents, vasodilatation also occurs in the corresponding deeper organ. Increased blood supply helps to fight the cause of pain and inflammation in the deeper organ.

 

Counterirritants are generally massaged to relieve headache, muscular pain (torticollis, backache, sprain), joint pain, pleural/peritoneal pain, colics, etc. Drugs are:

 

Volatile oils (essential oils) are terpene hydrocarbons of plant origin having a characteristic odour. They have variable properties, but all are irritants. Stearoptenes are solid volatile oils.

 

Turpentine oil Obtained by distilling Pinus oleoresin; used as counterirritant in the form of liniment or ‘stupes’.

 

Cloveoil Applied by cotton swab for toothache.

 

Eucalyptus oil Used in pain balms.

 

Camphor It is obtained from the bark of Cinnamomum camphora or produced synthetically. Produces cooling sensation on skin and is mildly anaesthetic—relieves itching. It is added in liniments and pain balms. Taken internally— small doses produce a warm and comforting sensation in epigastrium; large doses are emetic.

Systemically it produces excitement and convulsions (especially in children).

 

Thymol Obtained from Thymus vulgaris, has a pungent taste. It is included in pain balms.

 

Menthol From mint or prepared synthetically, has cooling and soothing action. It is added to pain balms, throat paints, throat lozenges and inhalers for relief of nasal congestion. It is also a carminative.

 

Mustard Seeds It contains a glycoside sinigrin and an enzyme myrosin. When ground seeds are soaked in water, myrosin hydrolyses sinigrin to release allyl isothiocyanate which is a strong irritant. Mustard plaster has been used as rubefacient and counterirritant. As a suspension in water 4–8 g, of ground seeds are emetic.                                            

 

Capsicum (Chillies) It is a powerful irritant, hot in taste. The active principle is capsaicin. It is a popular condiment in Indian cooking, and is included in some counterirritant preparations. After initial stimulation, capsaicin depletes afferent nerve endings of the transmitter substance P; may relieve postherpetic neuralgia on local application.

 

Canthridin A crystalline solid obtained from Spanish fly. It is a strong irritant, higher concentrations damage the epithelium and cause vesication—has been used to remove warts, etc. It is added to hair tonics—claimed to increase vascularity of scalp and promote hair growth.

 

Methyl salicylate (oil of wintergreen) In contrast to other salicylates, it is not used internally (induces vomiting, gastritis and systemic toxicity). It is combined with other irritants in liniments and ointments for muscle and joint pain.

 

Alcohol Produces rubefaction when rubbed on skin and is a vehicle for liniments.

 

Some Counterirritant Combinations

 

ALGIPAN: Capsicum oleoresin 0.1%, histamine 0.1%, methyl nicotinate 1%, glycol salicylate 5% cream.

 

ARJET SPRAY: Methyl salicylate 875 mg, menthol 1.6 g, camphor 1.5 g, benzyl nicotinate 20 mg, squalance 250 mg, glycol salicylate 875 mg per 50 ml spray.

 

EUTHERIA: Eucalyptol 7.2%, menthol 4.7%, methylsalicylate 11.25% balm.

 

MEDICREME: Methylsalicylate 8%, menthol 2%, adrenaline 0.03%, mephenesin 2.5%, chlorpheniramine 0.2%, cream.

 

RELAXYL: Capsicum oleoresin 0.05% mephenesin 10% methyl nicotinate 1% ointment.

 

VICKS VAPORUB: Menthol 2.8%, camphor 5.25%, thymol 0.1% turpentine oil 5.5% ointment.

 

IODEX: Methylsalicylate 5%, iodine 4% nonstaining ointment.

 

AMRUTANJAN: Eucalyptus oil 17%, camphor 10%, thymol 1%, menthol 4.5%, methylsalicylate 7% ointment.

 

CAPSIGYLD: Capsaicin 0.075%, methyl salicylate 20%, menthol 10%, camphor 5%, eucalyptus oil 5%, diclofenac 1% gel.

 

Contact Us, Privacy Policy, Terms and Compliant, DMCA Policy and Compliant

TH 2019 - 2022 pharmacy180.com; Developed by Therithal info.