Paracetamol for dogs - time for some fresh thinking

Paracetamol, the subject of this month’s module, is a very familiar drug. Most people have taken it at some time or other. We can buy it anywhere, and it’s also prescribed on the NHS in primary and secondary care. But we still don’t understand exactly how it works.

For dogs, paracetamol (in combination with codeine) has been available to buy without a prescription under the brand name Pardale-V since 1993. The name suggests that it is the veterinary version of a no-longer-marketed human medicine called Pardale (which contained paracetamol plus codeine and caffeine).

Availability of an authorised non-prescription veterinary medicine implies that the product has an acceptable risk-benefit profile and an acceptable level of safety when used in the absence of a veterinary diagnosis. However, you’ll see from the module that there is very little formal evidence on paracetamol’s efficacy or safety in dogs. Unfortunately we don’t know what evidence was presented in order to gain a marketing authorisation back in 1993 (it has only been a requirement for the regulatory authority to publish an assessment report since 2007). I’m guessing that a marketing authorisation was granted on the basis of established use.

This product, which might have seemed appropriate in 1993 doesn’t make sense now. Evidence shows that the codeine in Pardale-V is unlikely to have an analgesic effect in dogs because of poor bioavailability. Furthermore, it might adversely affect the absorption of paracetamol by slowing gastric transit, and it can cause adverse effects. Combination drugs should only be used if each component is necessary for the desired effect, and if the advantage over using a single drug outweighs the added risk of using two or more.

There is a difference of opinion among vets on what to use if they want to prescribe paracetamol for a dog. For some it means that Pardale-V, the only licensed form for dogs, must be used. Others see it differently: if paracetamol is the required drug, Pardale-V (because it contains codeine) is not a suitable choice, and so they go straight for human paracetamol. For those who prescribe Pardale-V, there is awkward information to negotiate in the summary of product characterstics (SPC), which suggests that paracetamol is an NSAID. This is not in keeping with the available evidence, and is inconsistent with the SPCs of (more-recently) authorised paracetamol products for pigs.

Another anomaly is that Pardale-V comes in pack sizes of 100 and 500 tablets. You can’t buy human paracetamol over the counter in packs of more than 32 (a restriction introduced to reduce the rate of deaths from paracetamol overdose). Why, if Pardale-V is only licensed for use for up to 5 days, is it available in such large quantities: the 100-tablet pack contains more than twice the number of tablets needed to treat the highest weight-category dog for 5 days.

Once a marketing authorisation for a veterinary medicine is granted, it is reviewed after the first 5 years, then provided there are no safety concerns, the product can remain on the market indefinitely. The regulatory authority responds to reports of adverse events to drugs, but changes needed to keep pace with the evidence do not happen systematically.

A recent article highlighted the difficulties the veterinary pharmaceutical industry faces in developing new medicines, because of the high cost of developing new chemical entities and because of loss of funding from human pharmaceutical companies as a result of industry restructuring. So maybe companies could look at ways of improving what we already have. How difficult would it be to produce a paracetamol product licensed for dogs backed by good evidence of efficacy and safety?

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