You can go through a pretty tedious process of crushing the pills and mixing them with coffee grounds or cat litter and putting them in the trash but there are better ways.
National Take Back Days
Take advantage of community drug take-back programs that allow the public to bring unused drugs to a central location for proper disposal. Call your city or county government's household trash and recycling service (see blue pages in phone book) to see if a take-back program is available in your community. The Drug Enforcement Administration, working with state and local law enforcement agencies, is sponsoring National Prescription Drug Take Back Days6 throughout the United States.
A few weeks ago while shopping; I came across TakeAway Environment Return System (there may be other such products on the market that I’m unaware of). I believe it was about $4 and includes a prepaid envelop that you use to mail your unwanted medications (even OTC) to this company and they dispose of them properly. You can’t mail controlled substances but most everything else they will accept. There may be similar products on the market. This company does make drop off boxes for pharmacies but I haven’t seen any yet. I think it’s a great concept for those of us that don’t usually make it to the Take Back Days.