The veterinarian put my dog on tetracycline to treat an infection. Someone told me that this antibiotic gave her cat all kinds of problems. Are there any side effects?

Yes, there are several potentially serious side effects with this antibiotic.

Tetracycline, oxytetracycline and doxycycline are the important tetracycline-class antibiotics used in veterinary medicine. The tetracyclines are bacteriostatic (they slow the growth of susceptible bacteria). They have anti-inflammatory properties as well. These drugs may be given by mouth, by injection, and by intravenous (IV) infusion. As with all medications, tetracycline antibiotics may produce side effects.

Tetracyclines can cause discoloration of the teeth and bones in young animals. This occurs because the drug concentrates in developing teeth and bones. The discoloration varies but may be grayish, brownish, or yellowish in color. For this reason, tetracyclines are not routinely given to young animals whose permanent teeth and bones are not fully developed. Generally, kittens and puppies may be given these antibiotics after six months of age.

High doses or long-term administration of tetracyclines can produce adverse physiologic effects. Tetracycline-class drugs may delay bone growth during fetal development, so they are not given to pregnant animals. They also can delay wound healing. High doses can interfere with protein formation and can be toxic to the liver and kidneys. Superinfection may occur with tetracycline usage. Superinfection occurs when the growth of tetracycline-sensitive, normally resident bacteria in the intestine is significantly slowed and non-sensitive bacteria overgrows and causes disease.

Even relatively average doses of the tetracyclines may produce side effects in some animal patients. The administration of these drugs can raise the effective levels of digoxin; animals being treated with digoxin and similar drugs should be monitored carefully while on tetracyclines. Likewise, animals on anticoagulants should be watched closely, since the tetracyclines decrease prothrombin function, and thus the ability to clot blood. Some veterinarians have observed low blood sugar in insulin-treated diabetics when given tetracycline.

The most common side effects of the tetracycline-class antibiotics in small animals include nausea, vomiting, diarrhea, and loss of appetite. Fever can occur that may be associated with undesirable stimulation of the immune system. Cats do not tolerate tetracycline and oxytetracycline very well. Cats can become colicky and can experience fever, depression and hair loss. Doxycycline may be better tolerated.

If a patient experiences side effects with a tetracycline antibiotic, when possible the drug is discontinued and the signs associated with the side effects are treated, if only supportively or symptomatically. The veterinarian will decide whether or not to substitute another antibiotic.

06/25/03