UNDENIABLY, Class 1 and 2 drug prevalence and violations are in decline in North America.
Most of these drugs are, for all intents and purposes, ill-intentioned when administered to enhance performance and the consequences on the welfare of the horse are destructive and far-reaching.
Nonetheless they continue to show up in drug testing albeit in lesser frequency than more "tolerable" medications.
Given the ambivalent nature of the questionable therapeutic use of Class 3 drugs it is worthwhile to consider two that remain on the current list of violations.
One such category of drugs is anabolic steroids, currently restricted but not truly banned from racing. Controlled by threshold limits they continue to surface in the countless violations that continue to beleaguer the industry. Anabolic steroids such as Boldenone or Stanozolol (Winstrol) are synthetic compounds that mimic the effects of male sex hormones.
By increasing protein synthesis within cells they effectively enhance the production of cellular tissue, particularly in muscles.  Most who administer these substances claim to do so to improve appetite, promote tissue healing and help the horse regain their competitive spirit. 
"Kentucky Derby winning trainer Bob Baffert comments. 'People complain that we don’t run our horses enough. If they get rid of steroids you’ll see them run even less often. If a horse has been racing hard or is dull, the steroids help recharge their batteries. They don’t make horses run faster'." 
Doubtful and hardly close to what these nasty drugs should be used for in the typical racehorse — if at all.
Baffert unsuccessfully endeavors to use the pathetic rationale of therapeutic medication but his statement clearly falters as it is in direct reference to enhancing the performance of the horse. Guilty as charged.
While it is quite likely that anabolic steroids may not improve the speed of the racehorse it is widely known that they do build muscle and can combat fatigue.
Moreover, horses in other parts of the world race very successfully without anabolic steroids. So why use them in North America?
According to many veterinarians there are significant gains in physical strength, stamina and mental attitude of race horses. 
So the bottom line again is simply for competitive advantage at the expense of the horse and the belief — clearly stated — that they improve performance.
In contrast to what these veterinarians assert, the negative aspects of steroid use are wide-ranging.
A horse’s muscle mass is primarily on the top half of their frame. At high speeds an enormous amount of weight comes down on their legs with only tendons and ligaments to absorb the huge amount of energy and power transferred to their limb structure. With an increase in body mass the risk to the Thoroughbred increases significantly given the fragility of their legs as a result of incessant inbreeding.
As for other effects on the horse, anabolic steroids can interfere with normal sexual functions rendering some of the stallions, geldings or mares who are administered them permanently infertile.
Moreover, these "roids" typically increase aggressive and unmanageable behavior creating a level of unpredictability that adds risk and danger to those who handle the horse.
"This behavioral inconsistency is pretty common with ABS or exogenous androgen treatment. It always rings true to me when I hear reports on the concerns about erratic behavior in humans on steroids — that's exactly my experience with horses. I'd rather be around a consistently tough stallion on his natural hormones than a mild stallion or a gelding or a mare that's on anabolic steroids. You let your guard down, and you can get hurt," McDonnell cautions." 
Steroids also contribute to clotting disorders, liver damage, heart attacks, strokes and weakened tendons. 
Apart from the persistence of steroid use, violations of the drug Clenbuterol over the permitted threshold are far more prevalent. Because it is a bronchodilator there has been great concern about its ability to enhance performance.
Sold under the name Ventipulmin, its pharmacological use should be restricted to horses with legitimate respiratory conditions. Instead, because it relaxes the muscles that line the airways, it is used in the otherwise healthy horse to increase the amount of air intake in each breath due to its ability to enlarge or dilate the airways.
By increasing the amount of oxygen to the lungs some believe that it will allow the horse to runner faster and enhance performance. However, it has been documented that in the healthy horse this is not the case.
"The drug was given intravenously to racehorses and oxygen levels were found to be no higher than in horses that did not receive it. Although it appears to help horses with diseased lungs, it does not seem to help healthy horses." 
While it is not an anabolic steroid, clenbuterol has some of the same effects such as increasing muscle mass and enhancing performance.  One of the more precarious effects on muscle function is that a horse tires more quickly than one not on the drug.
Could this contribute to breakdowns? Most definitely. Additionally it has been shown that long-term use has the potential of decreasing heart function, performance, exercise capacity and the horse’s ability to recover from physical exercise.
"After banning it as a performance enhancer, racing later permitted the widespread use of Clenbuterol — a drug originally marketed to fatten cattle — after its proponents claimed nothing else worked as well to clear out a horse's respiratory system. Despite evidence suggesting that this drug can alter the muscle mass of the heart, it is commonly used in racing." 
The question then arises as to whether the risks to the horse and the prospect of monetary penalty to the trainer are worth it. There is little doubt that the horse does not factor in as heavily as the potential profits. With the temptation to achieve full performance on race day and with penalties so low, there is no incentive not to.
 http://en.wikipedia.org/wiki/Anabolic_steroid  http://www.indianaharness.com/Announcement/IHRC/Anabolic%20Steroids%20in%20Horse%20Racing.pdf
 See footnote 4.
 See footnote 8.
Part 1: Introduction | Part 2: Historical Aspects | Part 3: The Inception of Drug Testing | Part 4: Drugs and Their Actions | Part 5: Policies and Tactics | Part 6: Class 3 Drugs — Performance Enhancing or Not? | Part 7: Class 4 Drugs — Harmless Therapeutics? | Part 8: The Unclassifieds | Part 9: The Call for Reform | Part 10: Who Rules?