What type of drugs may be adminstradted trandemally and why?

Transdermals are used to deliver a relatively continuous dose of a medication without having to constantly take oral medication. They are especially useful when treating patients who may be forgetful or have trouble keeping track of their doses (for one reason or another), or when administering drugs that could be harsh on the GI tract.

Lots of drugs have transdermal options. A few examples: narcotics (and other analgesics), nicotine patches, and contraceptives.