You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

Many therapeutic drugs can cause side-effects in patients. Sometimes it is difficult to decide whether the benefits of the treatment outweigh the side-effects that can result .

You probably don't know this, but every time you take an aspirin tablet your stomach bleeds, and you  lose about 1ml of blood. You don't know it is happening , you usually don't feel pain, and your perception of the therapeutic benefit of the aspirin is that it is good for you. Occasionally, patients taking aspirin have much more serious hemorrhage, and if the drug is taken for a long time you can develop gastric ulcers. In high doses aspirin can be life-threatening. 

Many drugs that have a beneficial effect within the body can have negative effects in certain patients - for a variety of reasons. Anti-inflammatory drugs may reduce the immune response making an individual more susceptible to infections. Antibiotics which kill one type of bacteria may encourage the growth of other bacteria or organisms such a candida - which can cause problems to the patient. Potent cytotoxic drugs that are used to kill cancer cells will also kill normal cells - causing unpleasant side-effects for the patient.

The veterinarian has to make a decision about which are the best drugs to use for an individual patient, and part of that decision-making process includes consideration of the potential benefits against the possible side-effects.

A correspondent to Provet recently related a case of a cat with hair loss which responded to treatment (ie the hair grew back), but the cat put on a lot of weight on the treatment. When the treatment was stopped the body weight returned to normal, but the hair thinned again. Another correspondent related the case of a dog with arthritis which developed gastrointestinal bleeding when treated with anti-inflammatory drugs. These are not uncommon problems. 

Sometimes it is possible to reduce the dose of a drug sufficiently to avoid the side-effects but retain the therapeutic benefits. In other situations a total change of drug is needed. If no other treatment works - a decision has to be made about whether to continue with treatment at all. . In the two cases above - gastrointestinal hemorrhage is serious and totally unacceptable so the drug can not be continued, in the case of hair loss it could be that the bald appearance of the cat is more acceptable than subjecting the patient to the long term side-effects of the drug.

The occurrence of side-effects to drugs is carefully monitored by regulatory authorities in the UK and your veterinarian has to report any suspect adverse reactions following the administration of a drug. So, if your pet does develop a reaction or possible side-effect to treatment be sure to tell your veterinarian. 

Finally, if your pet develops a problem soon after a drug has been given, don't be surprised, but don't assume that it is a side-effect because it could be totally unrelated to the drug . For all drugs the possible side-effects are well documented and your veterinarian will be able to advise you whether or not to discontinue treatment.

Updated October 2013