It's actually quite normal for those monstrously big blemishes to take a long, long time to completely disappear. Really deep or large pimples can take up to several months to flatten out and heal.
So, if you've been battling a big one for a while, you can rest easy knowing that it's not unusual to have a pimple that seemingly does not want to go away. Nodules (deep, hard, inflamed breakouts) and cysts are notoriously long-lived blemishes, but even large papules can take many weeks to disappear.
If you'd rather not wait for it to go away on its own, there are things you can do to speed up healing and shrink those "big guys" faster.
Don't PopFirst and foremost -- don't ever try to pop a deep pimple. Don't poke it with a needle. Don't squeeze it.
Squeezing, poking, prodding, or otherwise messing around with the blemish makes healing time all that much longer. It can also make that big pimple much bigger, and much worse.
Trying to pop this type of pimple can seriously damage your skin and cause scarring. As much as you might like to, don't try to drain this pimple yourself.
Treatment MedicationsIf you use topical acne treatments, continue to do so. You can even dab on a bit more on the monster pimple to help it heal more quickly. Don't confuse a dab with a big blob, though. Using too much topical medication too often will dry out and irritate the skin instead.
Full disclosure here: spot treating works best for pustules (pimples with a white head). It really won't do anything for those deep nodules or cysts.
If you break out regularly but don't typically use an acne treatment, now is a good time to start. Consistently using acne treatment medications stop pimples from forming before they have a chance to appear.
Warm CompressesYou can encourage the pimple to come to a head on its own by using a warm compress. Make one by soaking a washcloth in really warm water. Apply to the pimple for 20 minutes or so, a couple of times a day. You might have to do this for several days.
With any luck, the warm compress will help the pimple to drain on its own. Remember, though, don't squeeze at the pimple even if it comes to a head. Let it drain naturally.
Incision and DrainageWhen at-home treatments aren't helping, it's time to call in the pros. Those deep cystic breakouts often need to be lanced and drained.
The dermatologist will make a small incision in the skin and carefully drain the debris from the blemish. It's a bit uncomfortable, but not as bad as it sounds.
Cortisone InjectionsAnother pro treatment for big pimples is an intralesional cortisone injection.
A tiny needle is used to inject a dilute steroid into the blemish. Within just a few days, the pimple shrinks and disappears.
Again, this procedure must be done by a dermatologist.
Make Sure It's Really a PimpleAre you sure it's just a pimple? What you think is a pimple could really be something entirely different: a boil, infection, even skin cancer.
Any pimple-like bump that appears on your skin and just won't heal warrants a trip to the doctor. This is especially true if the bump is extremely large, inflamed, or painful, or if the pimple pops up in a place you typically wouldn't expect to find acne (your underarm, for example, or the back of your knee).
If you aren't 100% certain it's an acne breakout, please make an appointment with your physician.
Source: "Treating Severe Acne." AcneNet. American Academy of Dermatology, n.d. Web. 25 Jan 2013. Treating Severe Acne