Veterinary Prescriber is the most useful resource I’ve found for information and critical evaluation of everyday veterinary practice.”

Hazel Bentall, Veterinary Surgeon.

Why do this module?

You can't avoid promotional messages about veterinary medicines. They're everywhere - in periodicals, at congresses and CPD meetings, and in company rep visits to your practice. But you can prepare yourself to see beyond the claims and find out what you really need to know about medicines. This mini-module from Veterinary Prescriber is in three parts:

  • a 5-minute video that shows the pitfalls to look out for and how and where to check out the facts.

  • a read and/or listen to section on what to look out for when judging the reliability of clinical trial evidence.

  • a short quiz to test your knowledge. Submit your answers and we'll send you a certificate for your CPD records. 

This mini-module is an example of our unique multimedia modules that give you independent, practical, evidence-based information on veterinary medicines. We carefully design the modules so that they cover what you need to know in your daily practice and empower you to make rational prescribing decisions in the best interests of your patients. What's more, the information is succinct and in bite-sized chunks, designed to save you time (and money). There's a lot of free CPD material around, but if it's free who's paying for it and what might they want to sell you? Veterinary Prescriber is supported by subscription not sponsorship. Authoritative, independent veterinary medicines information.

Start by clicking the play button below and viewing the tutorial. You can use the player controls to pause, play, rewind and fast-forward.

Whether you’re discussing the evidence on a drug treatment with a company rep, or evaluating a randomised controlled trial report yourself, or reading a summary of clinical trial evidence (such as a Veterinary Prescriber article), it’s essential to understand the terms used and the key features of randomised controlled trials, including the potential sources of bias. Here is a guide to help you.

A randomised controlled trial is the best type of trial design to find out about the efficacy of a drug treatment. The design aims to ensure that the only difference between the two groups of subjects in the trial is the drug of interest, so that any changes can be attributed only to the drug.  

Use of a control. The trial needs to include a group of subjects that does not get the test treatment. This rules out the possibility that factors other than the treatment had anything to do with the outcomes. The way to find out if a treatment works is to compare it with a placebo control. If there is already a standard treatment, with known efficacy, then it can be more meaningful to compare the test drug with the standard drug (which would be an active control). The active comparator should be identical in appearance to the test drug if possible.

Randomisation. The treatment and the control are allocated randomly. This means that there is the same chance of a subject receiving the test treatment or the control. Randomisation ensures that a subject cannot be knowingly or subconsciously allocated a treatment. It also ensures that the groups are broadly identical apart from the test treatment so that any differences in outcome can be attributed to the test treatment.

A placebo is an intervention that matches the test treatment in every way possible, except that it does not contain the active drug (e.g. it can be an identical tablet or capsule containing the same inactive ingredients). Using a placebo helps to blind the study.

Blinding rules out subjective bias. The people measuring the outcomes must not be aware of (i.e. they are ‘blind to’) treatment allocation. Blinding is especially important if subjective outcomes (such as changes in behaviour) are being measured. If the animal owner is involved in reporting treatment effects they too should be blind to treatment allocation.In human trials, the term ‘double-blind’ is used when both the investigators and participants are blind to treatment allocation.

The trial report should make a clear statement about what the trial is setting out to do. The outcome measures (sometimes called endpoints) should be chosen to suit the objective of the trial. For the purpose of obtaining a product licence (also known as a marketing authorisation) for a drug, the choice of outcome measures is guided by the regulatory authorities.

Primary outcome measure: The main measure of the effect of the treatment is called the primary outcome measure or primary endpoint. It should be clinically relevant and directly related to the main goal of the trial. The sample size of the trial should be determined by power calculations involving the primary outcome measure.

Secondary outcome measures. Trials often look at the effects of a drug on outcomes other than the primary one; these are secondary outcomes. They should also be specified before the start of the trial and be included in the methods section of the report. Secondary outcome measures do not have the same statistical authority as the primary measures, and it is more likely that positive changes in secondary outcomes will be due to chance. It’s important not to put too much weight on a secondary outcome result, but see it as something interesting that requires proper testing.

The study should be adequately powered for the primary outcome measure and the results for all outcome measures should be reported.

It is important to look for the data on adverse effects (harms) in a trial because you would probably not want to use a treatment if the benefits were outweighed by harms. However,  clinical trials will only usually pick up the most common adverse effects. Randomised controlled trials are not usually big enough or long-lasting enough to detect effects that are rare or that only arise after long-term use.


If you prefer, you can listen to the whole audio presentation about Drug trials in the following podcast. Don't forget that you can also download the podcast to your iPod, music player, tablet or smartphone using the Download link on the right of the audio player.

Goal of activity: Update knowledge; to help improve critical appraisal skills.

Authors/disclosures: Veterinary Prescriber editorial team/no conflict of interest

Specific learning objectives: to improve knowledge and understanding of randomised controlled trials.

Click the button below when you are ready to answer the CPD questions based on the Drug trials module from Veterinary Prescriber.

Once complete you will be emailed with your certificate so make sure you enter your email address carefully at the head of the form.

Now just think how useful this is going to be next time you are faced with a pharmaceutical company rep quoting impressive statistics. Armed with this and so much more from Veterinary Prescriber, you can make rational prescribing decisions, keep up to date on medicines, and save time (and money) searching for the evidence to support your decisions. A subscription to Veterinary Prescriber costs £9.95 per month (cancel anytime). Practice subscriptions available too.