The general assembly of equestrian sport's international ruling body passed the progressive list of banned substances over the existing list of banned substances in a close vote last week.
The petition, at www.no-fei.com, is urging the FEI not to apply the progressive list. "It violates animal welfare to allow a horse to show in a competition although it needs painkillers. We request the FEI to immediately put their focus back on the welfare of the horse."
The petition's demands are also stated in German, French and Spanish.
The progressive list allows phenylbutazone (bute), up to 8 micrograms per millilitre in plasma or serum. This is three times the level permitted in the 1980s before the powerful anti-inflammatory agent was banned.
The progressive list also allows salicyclic acid (similar to aspirin) up to 750mcg/ml in urine and up to 6.5 mcg/ml in plasma or serum. Flunixin, a common anti-inflammatory and painkiller in horses, will be allowed up to 500 mcg/ml in plasma or serum.
The new rules allow for a horse with levels below the prescribed limits to pass a drugs test provided the drugs are not used in combination.
The new list also prescribes acceptable levels for acetycysteine, which is used for some respiratory conditions; dichloroacetate (lactanase), which helps prevent tying up by reducing the buildup of lactic acid in muscle cells; and isoxuprine, a blood vessel dilator often used in the treatment of hoof conditions.
The general assembly voted strongly in favour of the federation's Clean Sport strategy, which FEI president Princess Haya later hailed as a true landmark moment in the history of the sport.
However, most attention has since been focused on the progressive list, with major horse nations concerned over how sponsors will view the new stance on a set of drugs commonly used in horses.
Some have questioned how the industry can sell a "clean sport" image when it tolerates the presence of such key substances.
"The progressive list now permits in-competition use of a limited number of medications under strictly prescribed limits," the FEI said in a statement after the vote.
The assembly also approved an itemised list of more than 1000 substances not allowed in competitive horses.