Paracetamol for long term use

Paracetamol is useful as an alternative to NSAIDs in dogs that will not tolerate an NSAID or as an adjunct to NSAIDs in chronic pain. Paracetamol should never be used in cats.

Paracetamol has multiple effects

· The anti-inflammatory effect is weak

· It antagonises COX 3 centrally thus inhibiting CNS prostaglandins

· It is a serotonin agonist working via the descending inhibitory mechanisms

· Intact cannabinoid receptors are required for paracetamol’s actions

This evidence regarding mechanism of action is the justification for using paracetamol alongside NSAIDs.

Paracetamol is licensed in dogs as Pardale V for 5 days. The dose in Pardale V works out at 33mg/kg TID. The listed dose in most formularies is 10mg/kg BID-TID, although the origins of this recommendation are unclear. This discrepancy does lead to some uncertainty and confusion about how we should be dosing paracetamol for long term use in chronic pain. Other formulations of paracetamol which can we used via the cascade are paracetamol tablets and oral suspension. In the UK there is a veterinary special made by Bova UK.

What does the evidence say?

There is actually no evidence for the use of paracetamol long term beyond anecdotal use. The evidence is acute pain is growing and is reviewed in this webinar. Many of us are, quite rightly, using paracetamol as an analgesic for chronic pain and documenting efficacy based on owner reports of improved comfort.

Given that we have a huge body of evidence regarding safety and efficacy with NSAIDs in dogs, NSAIDs are always our first line option for chronic pain. This information relates to osteoarthritis (the biggest cause if chronic pain in dogs) but of course NSAIDs have a role in other pain aetiologies.

How do I use paracetamol?

• Pardale 33mg/kg TID 5 days

• Thereafter consider efficacy at reduced dose of 10-25mg/kg TID, but realistically 10 mg/kg is likely too low - for the discussion on this, view our webinar.

If you do reduce the dose and the owner considers comfort levels to have decreased then I will increase back to 25-33mg/kg and monitor. There is no evidence that this dose long term is detrimental. Remember paracetamol is hepatically metabolised (like all of our options for chronic pain) but this does not mean that it causes hepatic damage.

Justification for moving from the licensed product

I have seen cases where dogs have adverse events from Pardale V. Some owners describe behavioural alterations which could be attributed to the very low level of codeine in Pardale-V (although at 4% bioavailability this shouldn't have much effect). This gives us justification for using generic paracetamol (and ok, yes beyond 5 days we could use that same argument) and we then have a dose question. At this stage I would consider that you have a dose range from 10-33mg/kg and therefore some flexibility in finding a dose that suits your patient. Pain scoring is really key here and I would recommend using the Canine Brief Pain Inventory to document alterations in comfort.

Longer term monitoring

I always recommend monitoring a routine blood profile every 6-12 months in patients receiving multiple medications. To date I have seen few to no concerns with such patients although still consider this a sensible precaution. Of course in older patients we could be looking at natural progression of age related changes versus drug related - for drug related changes we would expect acute increases in hepatic and renal parameters to higher levels than those seen with insidious onset of age associated diseases. Any changes in such parameters then bring us to the 'what should we do next?' question in a patient with chronic pain where we often have good analgesic control. This is where I have the quality of life discussion with owners who mostly relay to me their desire to maintain comfort as a number one priority.

The above discussion gives reasonable options for using paracetamol long term. Consultation with the owner is important here as we need consent for any off label use via the prescribing cascade. In my experience 99% of owners are happy with such usage given that our aim is zero pain for as long as possible.

This pain update reviews the dose used in the acute setting.

This post was written by Matt Gurney.

Matt & Carl established Zero Pain Philosophy to provide educational resources to veterinary professionals enabling optimal management of pain.

Matt Gurney is an RCVS & European Specialist in Veterinary Anaesthesia & Analgesia and works at Anderson Moores Veterinary Specialists. Matt is President of the European College of Veterinary Anaesthesia & Analgesia.

Carl Bradbrook is an RCVS & European Specialist in Veterinary Anaesthesia & Analgesia and is President of the Association of Veterinary Anaesthetists. Carl works at Anderson Moores Veterinary Specialists.