AS MUCH as many drugs can and are used to heighten performance, by far the most controversial in the horse racing industry today are the Class 4 NSAID's (Non-Steroidal Anti- Inflammatory Drugs) corticosteroids and the much criticized "Lasix" (also known as Salix) — an unclassified permitted race day medication in North America.
It was in the 1970’s when North America adopted the policy of "permissive medications" that the administration of Lasix and painkiller Butazolidine — the brand name of Phenylbutazone (commonly referred to as "Bute") — became standard at the track. Hence there is a long history of drug abuse when it comes to these types of medications.
Many in the industry believe that Salix is beneficial to the horse because it prevents the common problem of bleeding in the lungs or EIPH (exercise-induced pulmonary hemorrhage).
The majority of the racing jurisdictions in North America also permit the administration of one of the NSAID's over a prescribed period of time (no earlier than 24 hours before race time).
These are typically either phenylbutazone ("bute"), flunixin or ketoprofen. Others include steroidal (catabolic) anti-inflammatory drugs or corticosteroids used to decrease inflammation in joints.
There is much concern that the presence of these drugs interfere with the ability of the tending veterinarian to properly evaluate the horses on race day as they have the tendency to mask lameness and respiratory problems.
In fact, over medicating with legal drugs together with lax oversight to their use has led to a consensus among equine researchers and surgeons that legal medications such as Salix and Bute as well as cortisone shots, over time, leave a horse vulnerable to a catastrophic breakdown. 
Bute and banamine (technically known as phenylbutazone and flunixin meglumine, respectively) are both non-steroidal anti-inflammatory drugs (NSAIDs). They and other NSAIDs operate not on the pain receptors in the brain but in the musculoskeletal tissue, where they control inflammation. With the reduction in inflammation comes pain relief.
The bandaged forelegs of Kentucky Derby and Preakness winner, I'll Have Another.
Intra-articular corticosteriods have been used for many years in the horse racing industry to aid in the treatment of the swelling and inflammation in the joints of horses.
These catabolic steroids act as potent anti-inflammatories and immuno-suppressants that have benefits in low dosages for therapeutic uses only. Typically these drugs are injected directly into the joints analogous to transmission fluid for an automobile.
"Used properly, corticosteroids can be beneficial at lower doses about six to seven days before a race, Soma said. But they often are administered 'improperly' — 48-72 hours before a race, in multiple joints, and perhaps weekly, he said." 
Although there are threshold limits to abide by their use is persistent and the wide-spread overages in higher than recommended doses in otherwise healthy horses contribute to a host of problems - in particular serious negative consequences on cartilage integrity. If used inappropriately, these powerful anti-inflammatories can destroy the cartilage and potentially contribute to chronic arthritis. 
Additionally the use of corticosteroids can delay healing of soft tissues such as ligaments and tendons meaning that if a horse is relying on the drugs to relieve pain and swelling the underlying injury is still subject to repeated stress that is now masked and as such can create further and more acute damage to the tissues. 
These are not the only side effects that have been documented.
Not only are the corticosteroids anti-inflammatory medications but they are also immuno-suppressants meaning that they have the capacity to suppress immune system function resulting in an increased susceptibility to infectious diseases and cancers.
When used on a frequent long-term basis many of these horses may be vulnerable to chronic infections, especially lung and sinus problems (e.g. viral respiratory illness). 
If that isn’t enough:
"Another side effect of corticosteroids in the horse is laminitis or founder. The specific reason why corticosteroids predispose to founder is unknown, but it is thought to be related to an effect on the blood supply to the deep layers of the foot. The occurrence of this phenomenon is variable and is more likely to occur when more potent types of corticosteroids are used or very high doses of the less potent ones are used for long periods of time.
The exception to this is the individual horse, which for unknown reasons, is more sensitive to corticosteroids with respect to founder. It has been observed that even a few low doses of less-potent corticosteroids will cause sore feet in some horses." 
And finally, it has been demonstrated that chronic and inappropriate use can lead to life-threatening systemic hormonal and metabolic changes. These include but are not limited to: weakness and loss of muscle mass, suppression of the ability to produce natural corticosteroids, depressed calcium and potassium levels, induced diabetes-like state with symptoms of increased thirst, hunger and urination and last but not least stomach and duodenal ulcers. 
These drugs have the potential of killing a horse at the wrong dose.
Obviously the boundaries of drug administration in the race horse have exceeded the wisdom of normalcy as well as ethical significance. As the industry is dependent on these horses, putting them at risk for the sake of financial gain is forthright insanity.
Horse suffering from Clostridial Myositis: gas gangrene.
When injected intra-muscularly, Banamine (Flunixin) can cause muscle damage. Spores of the bacteria Clostridium can lie dormant in muscle tissue and begin to proliferate if muscle is damaged. The growth of this bacteria causes Clostridial myositis (gas gangrene), a serious and sometimes fatal condition, where gas builds up under the skin at the site of the instra-muscular injection. Bacterial toxins are released into the bloodstream and horses quickly become very ill or toxic. If not treated immediately, this can lead to death.
Phenylbutazone — or "Bute" for short — was introduced into veterinary medicine in the 1950’s and by 1959 had been approved for use in racing by the State of Colorado.  Many believe that this was the beginning of the era of "permissible controlled substances" in the horse racing industry.
Probably one of the most famous cases of the use of "bute" in horse racing was the disqualification of Dancer’s Image in the 1968 Kentucky Derby.
Post-race urinalysis revealed trace amounts of Bute but, while at the time legal at some tracks, Churchill had yet to adopt this rule. Shortly thereafter it was legalized in most states and was well established in racing jurisdictions by the mid to late 70’s.
Many claim that Bute, currently one of the most common drugs used in horse racing, is akin to aspirin.
Classified as therapeutic there are many misconceptions about this hypothesis, primarily from the standpoint that it allows injured horses to continue training and running. Anti-inflammatory drugs do not cure musculoskeletal problems although they do provide control of inflammation which helps to decrease further damage while controlling the discomfort to the horse during painful episodes.
"Phenylbutazone seemed a miracle drug when the stuff began entering the bloodstreams of racehorses in the 1960s. I was collecting the post-race urine that concentrated the metabolites of that drug during the ’60s, and as a teenager I became acutely aware of drugs and racehorses.
What a soothing anti-inflammatory effect bute brought to racehorses in those simpler days when its use first became widespread. The alleviation of certain lamenesses was dramatic. 'Really sweet stuff,' I remember Wright Haggerty’s Kentucky groom telling me on the Shelby, Montana, backside in 1965 as he pestelled up tiny white 100-milligram dog pills he had received from my father, the attending and regulatory veterinarian (thus my job as urine catcher)." 
Phenylbutazone is used to treat a wide range of musculoskeletal disorders such as sprains and strains, muscular overuse (including both muscular damage and strain/damage of the tendons attaching muscles to bone), tendonitis, acute joint injury/strain/sprain, and arthritic conditions.
It is also often used to control the fever associated with viral or bacterial infections but its use under these circumstances may mask the severity of the problem.  No doubt that short-term relief is of merit but continued administration can lead to severe and problematic consequences for the horse.
The question then arises as to the performance-enhancing ability of this so-called innocuous "aspirin-like" medication. There is no doubt to this end effect.
Although Bute may not make a horse run faster, nonetheless it enables mobility via its anti-inflammatory effect. A horse that feels no pain will run as it would without the underlying physiological problems associated with their legs, feet or joints.
Weakened musculoskeletal structures however are prone to further injury when worked to the same extent as their healthy counterparts.
The other problem that’s lies within is the mindset that if the horse doesn’t respond as expected simply increase the dosage. It has been proven time and again that the proper management of lameness in the horse is rest. Unfortunately for the horse, the owners and trainers are not willing to wait these long periods of time as with every passing day they lose more money.
"With racehorses the clock is ticking, fast. If drugs can save time with racehorses, they are used for just that. And that is the case these days. The industry has transcended bute. The monthly veterinary bills at Belmont and Aqueduct often exceed the monthly training fee. Ask any owner." 
Unmistakably the use of Phenylbutazone and other common NSAID's such as flunixin or ketoprofen allows the horse to continue training or return to training in a shorter period of time.
However, with these drugs masking the degree of lameness that continues to exist, there is the consideration of the contribution to both catastrophic and non-catastrophic breakdowns.
This is especially true in the case of "stacking" NSAID's — a common practice which is routinely used and appears frequently in the record of violations. Combining different NSAID's is an illegal practice due to this masking effect on lameness which can, in effect, eliminate pain. It is callous and irresponsible to believe these medications do not contribute to further detriment to the horse.
As with the corticosteroids, Bute can also increase the propensity toward gastro-intestinal ulceration when administered on a continuing basis.
Gastric ulcers are common in horses, with prevalence estimated from 53 to 93%, depending on populations surveyed and type of athletic activity. The gastric ulcers in horses are caused by many factors including, anatomy of the stomach, diet, restricted feed intake, exercise, stress (stall or transport), and the use of non-steroidal anti-inflammatory agents.  In racehorses the incidence tends to the upper limit as a result of the implacable combination of strenuous exercise, stress and NSAID's.
This tendency of increased gastric ulceration however is neatly managed by yet more drugs. Enter the proton pump inhibitors (e.g. Ranitidine (Zantac), Omeprazole) which suppress the production of gastric acids – "Nexium" for horses.
It seems then that the Class 5 medications such as these serve as a category of drugs that manage the effects of the other more potent ones — plainly a domino effect and a never-ending cycle of drug abuse that ultimately shortens not only the racing careers of these guileless creatures but also their life expectancy.
In 2008, the U.S. House Subcommittee on Commerce, Trade, and Consumer Protection examined the state of horse racing.
During a hearing, Susan M. Stover, DVM, PhD, Diplomate of the American College of Veterinary Surgeons, Professor, JD Wheat Veterinary Orthopedic Research Laboratory, School of Veterinary Medicine, University of California, Davis, testified:
"Musculoskeletal injuries are the greatest cause of racehorse death and attrition" . . . . "From 1990 to 2006, an increasing trend was observed for injury rates. The proportion of Thoroughbred horses with a fatal musculoskeletal injury during racing and training has risen from approximately 3 horses to 5 horses per 1000 Thoroughbred race starts. The proportion of Thoroughbred racehorses with a fatal musculoskeletal injury has risen from 17 horses to 24 horses per 1000 Thoroughbred horses that started a race. Musculoskeletal injuries resulted in 19-33% of racehorses leaving training within a 3 month or less period in the United States." 
Dr. Stover explained "many catastrophic, fatal musculoskeletal injuries" result from pre-existing, less severe injuries that occur from "repetitive, overuse". In other words, horses continue to train and race despite injuries or damage which may be untreated or do not heal sufficiently. The result is further injury and damage, many times catastrophic or fatal. 
These comments together with the fact that the average number of starts per year for a racehorse has gone from over 11 to about 6 in 40 years beg the question as to how much of a role medication has played on the shortened careers and lives of the Thoroughbred. On the contrary Seabiscuit raced almost 40 times as a 2-year-old yet some still remain in denial.
"Trainer Ken McPeek suggested the Thoroughbred breeding industry is in part responsible for the decrease in the number of starts per horse. He said the economics of the business have made horses more valuable; therefore, they don't race as much, and are often retired early, mostly for breeding purposes.
"Horses are every bit as sound today," McPeek said. "You can't blame medication for the problem. I think there is a correlation between (number of starts) and (the breeding industry), but how significant it is, I'm not sure." 
Repeated and long-term use of medications leads to tolerance; over time larger and larger doses are required to produce the same effect. Alternatively the effectiveness of the drug begins to ebb if the dosage remains the same.
Building a tolerance for a drug exacerbates both drug abuse and dependency. Increased breakdowns, fewer starts and prolific drug use — there must be a connection.
No time like the present has it been more important to put an end to this abuse and debauchery.
 http://goo.gl/4xmmZT [NYT link inactive; use search box].
 http://www.stablemade.com/hproducts/drugs/Dexamethasone.htm], [http://www.horseracingofficial.com/default.asp?id=8.2&article=314
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?