What is the difference between topical and transdermal?

Answer: While both routes of administration involve getting the drug across the skin, a drug is considered to be delivered transdermally only if it has central effects.

Drugs can be delivered into the body through several different routes of administration - oral, intravenous, and inhalation being the most common. Although the skin is very effective at keeping chemicals out, some chemicals are able to pass through the skin. Among those chemicals that do, the words “topical” and “transdermal” may be used. 

These two words are often mistakenly used for one another, but they have different meanings.

Differences between topical and transdermal

Topical medications are the kinds that we imagine have a local effect on the person. For example, these are often creams, lotions, or ointments. The most common topical medications are used to treat conditions like acne or eczema. A topical medication like Tiger-Balm or Icy-Hot acts at local circuits only, and they are useful for relieving pain or inflammation. In all these cases, the drug does not diffuse very far. It only moves around the tissue underneath where it is applied and has no effect on the central nervous system.

Transdermal medications, however, cross the skin and get absorbed into the bloodstream. Because these chemicals can enter the bloodstream, they can have an effect on several organs, including the brain. The common transdermal medications we may think of include a motion sickness patch, a nicotine patch for people who are trying to quit smoking, or a fentanyl patch for pain. Transdermal medications are more often used for chronic conditions.

Transdermal medications are so valuable because they provide a controlled release of the drug over time. Topical medications are usually used for short-term relief whenever symptoms are local. 

Compared with other routes of administration, dosing with transdermal medications is relatively simple since the application only needs to happen once. Some patch formulations are designed to last for several days. Topical drugs may need to be applied a few times a day.