Medicating an injury requires common sense.  Pain is nature's way of alerting the brain that something is amiss within the body.  There are some injuries that should provoke conscious recognition that reaching for the bottle of pills must be accompanied by relative, or even absolute, rest (see: Five Problems You Should Not Run Through).  However, for most minor aches and pains, and sprains and strains, the average runner will attempt to persist, while subverting pain signals with aspirin, acetominophen, or anti-inflammatories.  We refer to these as "over-the counter" (OTC) medicines, i.e.- they can be purchased at the store without prescription. 

Aspirin is probably the cheapest and most traditional of the analgesic drugs.  Derived from willow bark and salisylic acid, it is the benchmark of the anti-inflammatory category of drugs.  Used sparingly and with caution, it is remarkably effective, and relatively safe, and costs less than candy.  It has three basic functions: relieving pain, reducing fever, and blocking the process of inflammation that produces four cardinal signs in injured and damaged tissues: redness, swelling, pain, and heat.  However, aspirin on on empty stomach, taken in large quantities, or used in subjects susceptible to stomach ulcers, can be disastrous.  Negative side effects include simple irritation of the stomach lining, "gastritis", or ulcers and bleeding, "peptic ulcer disease".  Bleeding rarely can be brisk enough to threaten one's life, but aspirin does pose such risk.  Two other serious side effects deserve mentioning.  Life-threatening allergic reactions ("anaphylaxis") can occur in certain people with asthma, and reversible ringing of the ears ("tinnitis") is associated with large doses of aspirin.  

Because of these risks, other non-aspirin substitutes have become increasingly popular, especially, acetominophen (Tylenol^TM).  Acetominophen has no such serious risks to stomach and intestine.  On the other hand, it has no anti-inflammatory effect, but remains an excellent pain-reliever and fever-reducer.  It's chief problem is possible liver damage when used at doses of more than four grams per day.  This is equivalent to eight Extra Strength Tylenol^TM, or about eleven standard tablets or capsules, per day.

Anti-inflammatory medications also include a category called non-steroidal anti-inflammatory drugs (NSAIDS).  These include ibuprofen (Advil^TM, Motrin^TM, etc.), and naproxen (Alleve^TM).  Like aspirin but unlike acetominophen, they have potent effects on reducing the symptoms and signs of inflammation, while also reducing pain.  They do carry the risk of stomach irritation and ulceration, though less so than aspirin.  They should be used very cautiously in people who have known sensitivity to these drugs or their relations, or in those who have had stomach problems in the past.  

Prescription versions of NSAIDS are numerous and varied. They are available at much higher dosages than their OTC cousins, and therefore, pose higher risk to the GI tract.  Two new exceptions include the highly marketed, popular favorites Vioxx^TM and Celebrex^TM.  These two, available by prescription, work via unique mechanisms which confer a much greater safety level than the classic NSAIDS, and are convenient in that they can be taken only once per day ("long- acting").  However, they are quite expensive (about $2.00/pill), so much so that some managed care companies have often severely restrict their use.  

Remember, if you choose to self-medicate, you are consciously choosing to dampen the body's warning systems.  Any persistent or progressive ache or pain should at some point beg the question as to the reason for its presence.  Secondly, any drug, no matter how inexpensive, or ubiquitous, has side-effects and risks associated with its use, that must be weighed against its benefits.