My dog is on vetalog for allergies. What are the side effects of this medication?

Vetalog is a type of steroid medication. The most noteable side effects (at least the ones you'll appreciate)are increased thirst, urination, and increased hunger. In some cases an adverse side effect can be GI upset (vomiting/ upset stomach) less common, but still possible are stomach ulcers. There usually aren't any behavioral side effects like are reported in some people. With prolonged use of some steroids you have to monitor liver function to assess any systemic effects. But used at an appropriate dose, these side effects are limited at most.
hope this helps!

Precautions

Vetalog Parenteral (Sterile Triamcinolone Acetonide Suspension USP) should not be used to alleviate pain or reduce inflammation arising from infectious states unless concomitant antimicrobial therapy is given.

Because of the anti-inflammatory action of corticosteroids, signs of infection may be hidden and it may be necessary to stop treatment until diagnosis is made.

Overdosage of some glucocorticoids may result in sodium retention, fluid retention, potassium loss and weight gains.

Corticosteroids have been used in the treatment of laminitis; Vetalog Parenteral (Sterile Triamcinolone Acetonide Suspension USP) is not recommended for that use. Cases of laminitis have been reported following the administration of Vetalog Parenteral; the mechanism of that response has not been fully elucidated. Care is necessary when using any corticosteroid in the equine species.

Use of corticosteroids, depending on dose, duration and specific steroid, may result in inhibition of endogenous steroid production following drug withdrawal. In patients presently receiving or recently withdrawn from systemic corticosteroid treatments, therapy with a rapidly acting corticosteroid should be considered in unusually stressful situations.
Adverse Reactions

As with any corticosteroid, polydipsia or polyuria may occur with high dosage or frequent administration of triamcinolone acetonide. The likelihood of their occurrence may be minimized by giving as brief a course of corticosteroid therapy as possible, and by waiting for the reappearance of symptoms before repeating therapy. If polydipsia or polyuria should occur, therapy should be discontinued until these unwanted effects have disappeared; therapy should then be resumed at a lower dosage level.

Other adverse reactions that have occurred with the use of corticosteroids are weight loss, anorexia and diarrhea (occasionally bloody). Anaphylactoid reactions have occasionally been seen following administration.

Intra-articular injection in leg injuries of the horse may produce osseous metaplasia.

Side effects such as serum alkaline phosphatase (SAP) and serum glutamic pyruvic transaminase (SGPT) enzyme elevations have occurred following use of synthetic corticosteroids in dogs.

Cushing's Syndrome in dogs has been reported in association with prolonged or repeated steroid therapy.