Chemistry in your cupboard

Nurofen Gel - a topical formulation

When taken by mouth (as tablets or caplets, for example) Nurofen works by being absorbed into the bloodstream and is carried to all parts of the body, not just the site of the pain. This of course is inefficient - the drug is only required at the site of the pain. It is also a potential source of side effects. As we have seen all NSAIDs may cause bleeding in the gut but, if you have a bad back, there is no need for any medicine in the gut, only the at site of the pain.

Topical application is the pharmacist‘s jargon for medicines that are applied to the skin as creams or gels. Nurofen is available as a gel that can be rubbed into painful parts of the body. This is advantageous in that the active ingredient is applied directly to the site of the pain rather than being spread over the whole body. It also helps to reduce side effects because the drug does not reach the parts of the body, such as the gut, where side effects may occur.

Figure 15: Nurofen Gel

The problem in formulating a topical application is getting the active ingredient through the skin. The outer layer of the skin consists of non-polar molecules and the inner layer of polar ones. Ibuprofen is, as we have seen, relatively non-polar and will therefore have difficulty penetrating the inner layer. This is because non-polar molecules tend to mix with other non-polar ones rather than polar ones. This is an example of the ‘like dissolves like’ rule.

So ibuprofen gel is formulated with propan-2-ol (isopropanol in the list of ingredients on the pack, Figure 16). This is of medium polarity. It is sufficiently polar to carry the ibuprofen through the inner layer of the skin but not so polar that it will not dissolve ibuprofen. Other ingredients such as hydroxymethylcellullose (a substance similar to starch) are used to make the product into a gel which will stay on the skin rather than running off.

Figure 16: Structure of propan-2-ol (isopropanol)