Medicines as Patches
EDP column by Dr Ketan Dhatariya

“I really don’t like taking tablets. My GP says I should be on all the time. Why can’t all my medications be given to me as a patch?"

There are an increasing number of drugs that can now be given as a patch. These include hormone replacement therapy, drugs for heart disease, Parkinson’s disease, motion sickness, attention deficit hyperactivity disorder, pain and so on.

The way a drug has to be delivered to the part of the body it is designed to treat is dependent on several factors. For example, for a drug to be able to be absorbed across the skin it must be quite potent. This is because of the ‘passive’ nature of absorption across the skin – if a drug was fairly weak, then not much of it would get across the skin and so not reach the levels in the blood required to do the job. Tablets, even of low doses, are often (but not always) absorbed very quickly across the very large surface area of the gut.

The next consideration is that the molecule the drug is made up of has to be small. This means that the chemical structure of the drug has to be small enough to allow it to get through the tiny pores in the skin and between the cells. If the molecules are too large, they will not get through the skin.

The next thing to consider is how soluble the drug is. If the drug does not easily dissolve in water, then it is less likely to be able to get through the cells in the skin. There is also a lot of fat in the lining of the cells so the drugs have to have the ability to dissolve in fat – as everyone knows, to try and get oil and water to mix is very difficult, so to get that balance right with a drug is very difficult as well.

Finally the drug must melt at a certain temperature. It is no use a drug being applied to the skin and it not liquefying.

It is very difficult for drugs to meet all of these requirements, although as you saw at the beginning, more and more are being developed. Don’t forget, however, that some people cannot use a patch because they develop allergic reactions to the adhesive, or even the drug itself, and so they have to take tablets.

As always, if you have trouble taking your medicines regularly, speak to your doctor of pharmacist who may be able to suggest an alternative way of taking it.