I am about to embark on a series of lectures, debates and discussions to promote the paperback publication of our book Trick or Treatment? Alternative Medicine on Trial. The first event is a public debate organised by King's College School of Medicine: "This house believes that complementary and alternative therapies do more harm than good."

One of the therapies under discussion will be homeopathy, and the evidence from clinical trials suggests that homeopathic pills are nothing more than placebos. Bearing in mind that homeopathic remedies are generally so diluted that they contain no active remedy, it seems obvious they can be nothing more than placebos. However, if previous outings are anything to go by, it will not be very long before someone at the King's College debate sticks up a hand and says: "Homeopathy must work, because it helped my pet cat!"

It is an interesting point, and one that sways many people who already have sympathies towards alternative medicine. After all, the placebo effect only works because the patient believes that a pill is supposed to be effective, and presumably Tiddles has no such belief system. So what is going on?

There are three possible explanations. First, if Tiddles has been conditioned to associate taking pills with improved health then it is conceivable that there is a placebo effect. Second, and more likely, it is possible that the owner selectively sees signs of recovery and falsely attributes them to the homeopathic pill, when they might have been due to natural healing processes or a conventional treatment that was happening in parallel. In other words, the owner is a biased observer. Third, we have to consider the unlikely possibility that homeopathy might be genuinely effective.

As it is World Veterinary Day tomorrow, let's celebrate the fact that hard-working veterinary researchers have been busy conducting lots of clinical trials to get to the bottom of the mystery. The evidence is not consistent, but a trend does develop if we focus on the better-quality trials.

For example, in 2003 the National Veterinary Institute in Sweden conducted a double-blind trial of the homeopathic remedy Podophyllum as a cure for diarrhoea in calves, and it found no evidence for the efficacy of homeopathy. In 2005, a Cambridge University research group conducted a double-blind trial to compare homeopathy against a negative control – a "dummy pill" – as a treatment for mastitis in 250 cows. An objective way of checking for any improvement in inflammation of the udder is to count the number of white blood cells in the cow's milk, and the conclusion was that homeopathy was no more effective than the negative control.

In short, the bad and unsurprising news for Tiddles is that homeopathy, with its lack of any active ingredient, does not work.

Nevertheless, a small fraction of vets endorse, promote and practise homeopathy, much to the frustration of their colleagues. Writing in the Veterinary Times in 2005, the Sussex vet Richard Edwards stated: "I used to be a sceptic. Now I am a cynical sceptic and I very much hope that the silent majority of this profession speak up soon and voice their own scepticism. We have all sworn to uphold the welfare of animals in our care, yet we continue to allow practices which prevent the application of conventional treatments, which are proven to work, in favour of remedies which are based on myth, faith and possibly deliberate fraud."

To highlight the problem, one group of vets established a spoof organisation called the British Veterinary Voodoo Society. They were particularly annoyed that the Royal College of Veterinary Surgeons was willing to publish an official list of homeopathic vets, thereby giving undeserved credibility to homeopathic remedies.

The only consolation is that homeopathy can only be practised on animals by a fully trained vet or under the supervision of one, because it is forbidden for the average high street homeopath to treat animals on their own. So Tiddles should be safe from the worst excesses of homeopathy.

Unfortunately, humans are not as lucky as pets. High street homeopaths, who typically do not have any serious medical training, are allowed to treat you and me for almost any condition. Indeed, it is an astonishing and shocking fact that anybody reading this column could call himself or herself a homeopath, create a fancy sign, put it on the front door, stick an advert in the local newspaper and start treating people.

Perhaps one day humans will get the same protection from homeopaths that animals already have.