Two thirds of acute ruptures of the Achilles tendon are associated with evidence of gradual pathologic changes within the tendon over time. For this reason you shouldn't attempt to run if you find the pain severe enough to alter your form.
If after cutting back on your training you have Achilles pain that won't disappear, see a doctor. Lest you face years of frequent reinjury and lackluster running performance, it's best to err on the side of caution. There are a variety of clinical and laboratory tests that can diagnose your problem; MRIs are a common and often vital way to evaluate the severity of a tendon injury.
Treatment
Avoid cortisone shots to treat tendinitis. Cortisone, while useful for treating bursitis, disrupts the beneficial inflammatory response essential to the healing process and therefore shouldn't be injected into tendon.
NSAIDs can help control pain in the short term but should be used judiciously so as not to completely shut down this inflammation necessary in the early healing stages. Obtain as many details as possible from your physician regarding the strength, dosage, and any long-term-use side effects of the particular NSAID or COX-2 inhibitor you may be considering taking.
Additionally, ice, topical analgesics and electrical stimulation have all been widely used to control the pain associated with tendon injuries.
The reversal of tendinosis depends greatly upon preservation of motion in the tendon and the gradual build-up of strength and endurance. Resistive exercise, stretching, and incrementally increasing aerobic activity are the key components of effective tendon rehabilitation.
Since the goal of rehabilitation is to align the fibrous collagen structures along the patterns most structurally beneficial for a particular runner's form, perform activities that simulate the motions of the tendon during your running -- so long as they don't exceed the tendon's capacity to resist force.
Be sure your doctor provides you light weight-training exercises that move the tendon appropriately. Deep-water running is another way to keep up aerobic capacity and simulate the movement of training on the roads without adding impact forces to injured tissue.
Bracing and orthotics are sometimes used to help prevent reinjury. Try to correct any abnormalities in your running form that may have contributed to your injury, and closely monitor your intensity and frequency of training in a diary, always heeding painful warning signs. In extreme situations, surgery can remove damaged tissue and repair the remaining tendon structures.
(Textbook of Running Medicine, "Tendinosis and Tendon Injury," 2001, McGraw-Hill, New York, pp. 37-43)