Diclofenac is sold as Voltaren in US and as Voveran in Canada. It's available in lower doses in US without a prescription. It is a "non-steroidal-anti-inflammatory-drug" (NSAID), in the same class as ibuprofen (Motrin, and others), indomethicin (Indocin) and even butazolidine
The most common side effects of this class of drugs are stomach irritation and occasionally bleeding; rarer but more serious side effects are liver failure (especially in large doses acutely) and kidney failure (in large cumulative doses over long times).
And keep in mind that words like "large" doses and "long" periods of time are relative, and may vary greatly from person to person, reinforcing the position that the "right" dose of most medicines is "as little as you can get away with..." (Not all of them; there's another bunch whose dose is predicated on objective markers, and for these, instructions should be followed strictly!)
