Can you take penicillin after the expired date?

In the US, expiration dates on pharmaceuticals tend to be set fairly conservatively... most drugs are still "good" well past their expiration dates, though they may be slightly less effective.

An FDA study conducted at the behest of the US Military found that 90% of the drugs tested were still useful and effective as much as 15 years (!) past their expiration dates.

The exceptions to this general rule are antibiotics (especially liquid antibiotics), protein-based drugs (e.g. insulin), and other quickly-deteriorating compounds like nitroglycerin.

Penicillin, as an antibiotic, is extremely questionable. You shouldn't have any penicillin lying around anyway, since you should ALWAYS take the FULL course of any antibiotics prescribed for you (failure to do so is why we've got so many strains of antibiotic-resistant germs around now). If it's two days past the expiration date and you've got two doses left, go ahead and take them. If they're from when your mom was sick six months ago, throw it away (better yet, give it to a physician or pharmacist for proper disposal), get your own, and take all of it.