57. Adjuvant analgesics. Centrally-acting muscle relaxants

Page created on June 13, 2019. Last updated on January 7, 2022 at 22:27

Adjuvant analgesics for neuropathic pain

Neuropathic pain is caused by damage to peripheral or central pathways. This pain is not easily treated as it responds poorly to conventional analgesics like NSAIDs, and even to opioids. Other drug types are necessary.

The following drugs are effective for treating neuropathic pain:

  • Tricyclic antidepressants
    • Amitriptyline
    • Nortriptyline
  • Antiepileptics
    • Gabapentin
    • Carbamazepine
    • Lamotrigine
  • Local anaesthetics
  • Capsaicin
  • Nabiximols (cannabis preparations)
  • Glucocorticoids

Nabiximols are described in “centrally acting muscle relaxants”. The other drugs have been described in other topics.


Capsaicin is the molecule in spicy foods that make them spicy.


Neuropathic pain.

Mechanism of action:

Capsaicin binds to receptors in nociceptive nerve endings and reversibly damages them, preventing them from causing neuropathic pain.


It’s applied as a dermal patch. It should be placed on the painful area for one hour. It takes 1 – 2 weeks for the analgesic effect to develop, but it lasts for up to 3 months.

Side effects:

When placing the patch there is a painful response, but this can be alleviated with local anaesthetics.

Centrally acting skeletal muscle relaxants

These drugs act on the CNS to reduce pathologically increased muscle tone, without compromising voluntary muscle contractions (like peripheral muscle relaxants do). This is relevant in two cases:

  • Spasticity – resistance to movement, involuntary muscle spasms and enhanced stretch reflex
    • Caused by a lesion of the upper motor neuron, which regulate the stretch reflex
  • Acute muscle spasms


  • Baclofen
  • Nabiximols (cannabis extract)
  • Benzodiazepines
  • Tizanidine


Spasticity associated with stroke, cerebral palsy, multiple sclerosis. Muscle spasms for any reason.

Mechanism of action:

Baclofen is a GABAB receptor agonist. Benzodiazepines bind to an allosteric site on GABAA receptors, stimulating them. Stimulation of GABA receptors inhibits the stretch reflex, which reduces muscle tone and spasticity.

Nabiximols bind to and activate cannabinoid receptors, which inhibit the stretch reflex.

Tizanidine is an α2 receptor agonist which inhibits the stretch reflex.

Peripherally acting agents used in spasticity and muscle spasms

Dantrolene and botulinum toxin are also used to treat spasticity.

Mechanism of action:

Dantrolene blocks the ryanodine receptor, which the release of Ca2+ from the sarcoplasmic reticulum.

Botulinum toxin irreversibly inhibits release of acetylcholine in the neuromuscular junction.


Dantrolene is given orally or IV. Botulinum toxin is injected into muscles.

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58. Drug abuse and dependence: general principles, opioids, anti-anxiety and hypnotic drugs, inhalants, ethanol

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