With all of the new topical compounds in the cannabis market, how do you choose the best product for you?

First let’s understand skin permeability. The skin is your body’s largest organ and provides a huge surface area to absorb cannabinoids. The stratum corneum (SC) is the top layer that forms a protective barrier to absorption into the lower skin layers and blood stream. The SC is made up of keratinocytes surrounded by a lipid matrix. Think of this as a brick and mortar structure with the keratinocytes making up the bricks and the lipid molecules as the mortar. The SC has 10-30 layers of these bricks with thicker layers on the palms of your hands and bottom of your feet. The areas with the thinnest SC layers, around the forehead, jaw, scalp, armpit, and for guys the scrotum, are also the easiest places for a chemical to penetrate the SC.

Your skin isn’t sponge and not many things can migrate past it into your bloodstream. Keep in mind that the SC is a protective barrier meant to keep out harmful toxins and protect the lower skin layers from infection. The SC also regulates the passage of water across the skin through keratin proteins that help keep the skin hydrated. Water is a critical component to aid absorption into the skin. Plump hydrated skins cells facilitate the uptake of therapeutic ingredients in skin creams.

How well an ingredient gets past your skin barrier depends on its size and molecular structure. Very small molecules can diffuse across the SC. Molecules with a similar structure to the lipids that make up the mortar between skin cells can solvate and diffuse across the skin. In chemistry jargon it’s said, “Like dissolves like”. These processes are slow and most compounds stay on top of the skin.

So how do transdermal creams or oils differ from other topical creams? The skin has it’s own system of cannabinoid receptors that interact with phytocannabinoids like CBD and THC. Most cannabis topicals will interact with the skin’s endocannabinoid system to help relieve pain and inflammation. There are limitations. Many topical creams only go skin deep as they use emollients like waxes and butters that sit on top of the skin. Compounds like beeswax, shea butter, lanolin, and cocoa butter are great at keeping the skin soft and preventing dehydration, but not so great when it comes to transport of active ingredients across the skin. These topical creams sit on top of the skin and offer localized relief from pain and inflammation.

In contrast, transdermal compounds go past the SC and into the bloodstream, having a wider impact across the whole body. Transdermal patches and creams are a means to bypass the 1st pass drug metabolism in the gastrointestinal tract. They’re also used as an alternative to those who have trouble taking drugs orally. Due to an enhanced skin penetration, transdermal products can use much lower concentrations of CBD and still be effective.

There are 3 ways ingredients can migrate across the SC that depend in the compounds size and whether it’s a water-loving (hydrophilic) or fat-loving (lipophilic) compound. The transdermal paths are as follows.

• Diffusion across the skin cells. Keratinocyte cells (the bricks of our skin wall) are porous and will allow hydrophilic compounds to diffuse across the cell. However, the cells need to be well hydrated for this process to work well. Dry skin whether from aging, living in a dry climate or over-exposure to sun will slow down this process. For this reason it’s important for a topical compound to contain water and humectants to draw water into the skin. Another factor of water-soluble compounds is pH. The pH of human skin is slightly acidic and ranges from 4.5 to 5.5. Topical creams with a pH outside this range will irritate the skin.

• Diffusion through the lipid matrix. This process involves diffusion of a lipophilic compound through the lipid matrix. This lipids that make up the mortar between cells is tightly packed and hard to penetrate. Molecules that are small and have a similar structure to the SC lipids (other lipids, phospholipids and fatty acids are a few) can diffuse through the SC via this route.

• Diffusion using a penetration enhancer. In the pharmaceutical and cosmetic fields these permeation enhancers are known as transdermal excipients. They help drive compounds like cannabinoids across the SC into the bloodstream and muscle tissue. Permeation enhancers work by temporarily breaking up the tightly-packed structure of the intercellular lipids to allow a faster transport of other lipid compounds such as cannabinoids across the SC. These permeation enhancers are widely used in medicinal patches for a slow, sustained delivery or in topical gels and creams for a faster onset. Most of these permeation enhancers are synthetic, but some are found naturally, such as water, alcohols, essential oils and oleic acid, which is the main ingredient in olive oil.

Finding the right transdermal cream, roll-on or patch depends on the application. Patches have the advantage of delivering a sustained dose over a long period and work well for ongoing chronic pain. Roll-ons typically have higher CBD concentrations and work quickly, but for a shorter duration. The thin SC on the forehead, temples, scalp and neck are great places to get fast relief from a transdermal product. This is what makes transdermal creams and roll-ons the product of choice for migraines and insomnia. For migraines and headaches the roll-on oil can be applied directly to the area of the head with pain. For insomnia a quick application of the roll-on oil across the forehead temples and tight neck muscles helps relax you so you can fall to sleep. Some roll-ons have added essential oils such as valerian for sleep, peppermint and lavender for migraines and arnica for pain. Transdermal creams are meant to cover a larger area and penetrate deep to the muscle tissue. Look for a transdermal cream that uses a water-based gel and a combination of penetration enhancers for the best results.