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aa BDS M6 - Therapeutics > 4. Analgesia - Paracetamol > Flashcards

4. Analgesia - Paracetamol Flashcards

1
Q

What is paracetamol (acetaminophen) a derivative of?

A

analine derivative

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2
Q

What 3 things are analine derivatives?

A
  • phenacetin, acetanalid and acetaminophen
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3
Q

What are the properties of the analine derivatives?

A

analgesic
- mild-moderate pain (similar to aspirin)
- anti-pyretic (similar to aspirin)
- anti-inflammatory property

one of the most common non-narcotic analgesics

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4
Q

What is the mode of action of paracetamol?

A

not very clear or understood
- suggested to have a more peripheral than central effect
- limited inhibition of COX-1/2 activity
- more highly selective an inhibitor of COX-3 but this is debated

other suggested modes of actions include:
- selective inhibition OG neuronal PG synthesis
- activation serotonergic pathways in spine
- inhibition of nitric oxide synthase

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5
Q

What effects (or lack of effects) does paracetamol have?

A
  • No effects on platelet aggregation
  • Little effect respiratory
  • Little effect cardiovascular
  • Reportedly fewer ADR’s (cf NSAIDs)
  • Little or no gastric irritation
  • No relevant impact on uric acid excretion
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6
Q

What are the indications of paracetamol?

A
  • first line for mild-moderate dental pain
  • inflammatory conditions
  • where NSAIDs are contraindicated
  • states - GI, renal, coagulopathies
  • drugs/conditions contraindicate NSAIDs - 4 ANTS
  • ?pre-emptive
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7
Q

What forms can paracetamol come in?

A

tablet, capsule, elixir, suppository, IV

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8
Q

How is paracetamol absorbed?

A

well absorbed in the GI (1st Pass limited)

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9
Q

What is the distribution of paracetamol?

A

widely and evenly distributed

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10
Q

What is the peak plasma time of paracetamol, and the % bioavailability of this?

A

30-60mins, bioavailability of 70%+

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11
Q

What is the half life of paracetamol?

A

2-4 hours
(4-8hrs in toxic doses)

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12
Q

Where does biotransformation of paracetamol occur?

A

liver

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13
Q

What is a possibly hazardous metabolite of paracetamol?

A

N-acetyl-p-benzoquinone imine (NAPQI)

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14
Q

How is paracetamol eliminated?

A

renal

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15
Q

What are the contraindications for paracetamol?

A

Few absolute contraindications, but some conditions which require special consideration

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16
Q

What are the side effects of paracetamol?

A

rare
- skin reactions (SJS, TEN)
- blood dyscrasias (thrombocyto-, neutro-, leuco-penia)

17
Q

What are the potential risks of long-term therapeutic paracetamol use?

A
  • liver damage (hepatotoxicity)
18
Q

Why must care be taken when dosing paracetamol for a patient on warfarin?

A

although general considered safe, some reports of significant alterations in INR and bleeding

19
Q

What may be done when prescribing paracetamol for a patient with renal insufficiency?

A

increase the dosing interval (i.e. more than 6hrs between doses)

20
Q

What situations are not absolute contraindications for paracetamol but require some care and consideration?

A

liver disease, alcohol use, malnutrition, dehydration

21
Q

What is the standard adult dose or paracetamol?

A

10-15mg/kg every 4-6 hours to a daily maximum of 4 grams (8 500mg tablets/day / 1mg 4x/day)

22
Q

What are the principle interactions with paracetamol?

A

beware (CYP450 2E1/1A2/3A4) inducing drugs: (SCRAPPP)-NAPQI:
- St. john’s Wort
- Carbemazepine
- Rifampicin
- Alcohol
- Phenobarbitol
- Phenytoin
- Primidone

23
Q

How do the SCRAPPP drugs interact with paracetamol?

A
  • the majority of the absorbed dose of paracetamol is metabolised by the glucaronidation and sulfation pathways to form non-toxic metabolites which are then excreted in the urine
  • the remainder is then oxidised by the hepatic CYP450 pathway, mainly CYP450 2E1/1A2/3A4
  • for patients taking enzyme inducing drugs (SCRAPPP) or those who are malnourished may develop liver toxicity at lower plasma concentrations and should be provisionally regarded as slightly higher risk
  • more NAPQI builds up at lower plasma concentrations of paracetamol if taking enzyme inducing drugs
24
Q

Is paracetamol effective for lower back pain?

A

no analgesic effect, in a cochrane review paracetamol’s effect was equal to that of the placebo

25
Q

What are the side effects of paracetamol?

A

A systematic review of (observational studies) shows paracetamol;
- Increased mortality
- Cardiovascular adverse events (fatal or non-fatal myocardial infarction, stroke, or fatal coronary heart disease)
- Gastrointestinal adverse events (ulcers and complications such as upper gastrointestinal haemorrhage)
- Renal impairment
- Non-overdose paracetamol = 2x rate acute liver failure (transplantation) than NSAIDs.
- Paracetamol = 4x more likely to have abnormal LFTs v placebo.
- RCT arthritis - paracetamol = ibuprofen for SE’s over three months.

26
Q

What amount of paracetamol is a highly toxic dose?

A

10-15g (20-30 tablets)

(can be as little as 8g depending on risk factors)

27
Q

What amount of paracetamol is likely to be fatal regardless of treatment?

A

25g

28
Q

When does maximal liver damage from paracetamol occur?

A

3-4 days post ingestion

29
Q

What is the treatment for paracetamol overdose?

A

acetylcystein IV, activated charcoal

30
Q

Summary slide (paracetamol):

A
  • non-opioid analgesic drug
  • analine dye derivative
  • first-line for most forms of acute and chronic pain
  • useful for mild-moderate dental pain
  • first step in WHO analgesic ladder
  • anti-pyretic activity
  • uncertain mechanism of action but likely a weak inhibitor of cytooxygenase
  • primarily oral administration but others available
  • side effects few, notable are skin reactions and -penias
  • liver metabolism, renal excretion
  • interactions: CYP450 inducers (SCRAPPP)
  • not entirely safe, commonly implicated in overdose
  • very few absolute contra-indications

aa BDS M6 - Therapeutics (6 decks)

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