The Skin: A Formidable Barrier
The primary function of the skin, the body's largest organ, is to act as a protective barrier against the outside world [1.2.3, 1.9.2]. It is composed of multiple layers, with the outermost layer, the stratum corneum, being the main obstacle to absorption [1.3.3, 1.4.4]. This layer of dead skin cells and lipids is designed to keep substances out, making it surprisingly difficult for most ingredients to pass through [1.2.2]. The common myth that 60% of what you put on your skin is absorbed into the bloodstream is largely inaccurate for most cosmetic products [1.2.2, 1.9.1]. Most skincare is designed to penetrate the upper layers of the skin (the epidermis) to exert a local effect, not to be absorbed into the systemic circulation [1.2.2].
Percutaneous Absorption: How It Works
When a substance does pass through the skin layers to reach the bloodstream, this process is called percutaneous absorption or systemic absorption [1.4.1, 1.5.1]. For a drug to achieve this, it must overcome the stratum corneum and diffuse through the viable epidermis to reach the blood vessels in the dermis layer [1.4.4]. This can happen through several mechanisms, including passive diffusion, where the drug moves from a high concentration on the skin to a lower concentration in the body [1.3.3]. The appendageal pathway, which involves absorption through hair follicles and sweat glands, provides another route [1.3.3]. While most topical products result in very little, if any, systemic absorption, some are specifically designed for it [1.2.1].
Factors Influencing Systemic Absorption
Whether a topical cream enters the bloodstream depends on a variety of physiological and physicochemical factors [1.3.1].
- Drug Properties: Smaller molecules with high lipid solubility (lipophilic) and a non-ionized charge are more easily absorbed because they can more readily pass through the lipid-rich stratum corneum [1.3.3, 1.3.4]. The concentration of the drug also plays a role [1.3.6].
- Skin Condition and Location: The skin's integrity is crucial. Absorption is significantly increased in skin that is damaged, burned, inflamed, or affected by conditions like eczema or psoriasis [1.3.2, 1.4.4]. Skin thickness also varies across the body; thinner skin (e.g., face, scalp, armpits) is more permeable than thicker skin (e.g., palms, soles) [1.3.3, 1.5.6].
- Formulation and Application: The vehicle or base of the cream (e.g., ointment, gel) affects absorption [1.3.4]. Some formulations include penetration enhancers, like alcohols or fatty acids, which temporarily disrupt the skin barrier to allow ingredients to pass through more easily [1.4.1, 1.9.4]. Occlusion, such as covering the application area with a bandage, increases skin hydration and can dramatically boost absorption [1.3.2].
Local vs. Systemic Effects: A Tale of Two Cream Types
It's important to distinguish between products designed for local action and those designed for systemic action [1.2.6].
Topical Products (Local Effect): Most creams, including many over-the-counter (OTC) hydrocortisone or antifungal creams, are intended to work locally on the skin's surface or within the epidermis [1.2.6]. The goal is to treat a specific area while minimizing impact on the rest of the body [1.2.6].
Transdermal Products (Systemic Effect): In contrast, transdermal drug delivery systems are specifically engineered to deliver medication into the bloodstream for a systemic effect [1.2.6]. These often come as patches but can also be creams or gels. Examples include:
- Nicotine patches for smoking cessation [1.8.2]
- Hormone replacement therapies (estrogen, testosterone) [1.8.2]
- Fentanyl patches for chronic pain management [1.8.3]
- Nitroglycerin ointment for angina [1.4.4]
- Scopolamine for motion sickness [1.8.3]
| Feature | Topical Creams (Local Effect) | Transdermal Systems (Systemic Effect) |
|---|---|---|
| Primary Goal | Treat a specific, localized area on the skin [1.2.6]. | Deliver a drug to the entire body via the bloodstream [1.2.6]. |
| Absorption | Minimal systemic absorption intended [1.2.1]. | Designed for significant systemic absorption [1.4.4]. |
| Examples | Anti-fungal creams, hydrocortisone for rashes, acne treatments. | Nicotine patches, hormone creams, pain-relief patches [1.8.2]. |
| Formulation | Works on the epidermis or dermis [1.2.2]. | Formulated with enhancers to cross skin layers into capillaries [1.3.4, 1.8.3]. |
Regulatory Scrutiny and Safety
Regulatory bodies like the U.S. Food and Drug Administration (FDA) require drug developers to assess systemic absorption, even for products intended for local effects [1.4.1]. The FDA's Maximal Usage Trial (MUsT) is a standardized study to determine the potential for systemic exposure under conditions of maximal use [1.7.1, 1.7.2]. This is crucial because unintended systemic absorption can lead to adverse effects. For example, potent topical corticosteroids applied over large areas or for prolonged periods can be absorbed in sufficient amounts to cause systemic side effects like adrenal suppression [1.4.1, 1.4.5]. Similarly, using a topical NSAID cream while taking oral NSAIDs can potentially lead to an excessive dose in the bloodstream [1.4.4].
Even some cosmetic ingredients, like retinoids or certain chemical sunscreen filters (e.g., oxybenzone), have been shown to be absorbed into the bloodstream, though the long-term health implications are still under study [1.9.4].
Conclusion
So, do topical creams get absorbed into the bloodstream? The answer is yes, but it is highly dependent on the product's design and other factors. While most cosmetic and many OTC medicated creams have minimal systemic absorption, transdermal medications are specifically designed to enter the bloodstream. Factors like molecular size, skin integrity, application site, and the presence of penetration enhancers all play a critical role. Understanding this distinction is key to using topical products safely and effectively. Always follow product instructions and consult a healthcare professional about potential risks, especially if you have compromised skin or are using multiple medications [1.3.3].
For more information on drug development and safety, you can visit the U.S. Food and Drug Administration (FDA).
