the optimal bioavailability and therapeutic effects. In case of overdose or unwanted side effects, patients can take off or wash out the medicines quickly to eliminate toxicity by simply removing the patch to stop the delivery of drugs. Disadvantages of topical drug delivery systems The site of putting the patches for topical drugs may get irritated and have rashes and feel itchy. Hence, some topical drugs including nicotine patches for smoking cessation are advised to change places for each application to avoid continuous irritation of the skin. Also, since the drug needs to penetrate the skin, some drugs may not be able to pass through the skin. Some drugs are then “wasted” and the bioavailability of the drug will decrease. Challenges for designing topical dosage form Skin penetration is the main challenge for any topical dosage form. The drug needs to penetrate the skin in order to get into the body to apply its function. The drug follows the Fick’s first law of diffusion. One of the most common versions of Fick’s first law of diffusion is: where is the diffusion flux. is the diffusion coefficient. is the concentration gradient. For is described by the Stokes–Einstein equation. The equation is: where is the gas constant. is the temperature. is the viscosity. is the radius of the solute. is the Avogadro constant. Assuming concentration gradient is constant for all newly applied topical drugs and the temperature is constant (normal body temperature: 37 °C), the viscosity and radius of the drug determine the flux of diffusion. The higher the viscosity or the larger the radius of the drug is, the lower the diffusion flux of the drug is. New developments There are many factors for drug developers to consider in developing new topical formulations. The first one is the effect of the vehicle. The medium to carry the topical drugs can affect the penetration of the drug active ingredient and efficacy. For example, this carrier can have a cooling, drying, emollient or protective action to suit the required conditions of the application site such as applying gel or lotion for hairy areas. Meanwhile, scientists need to match the type of preparation with the type of lesions. For example, they need to avoid oily ointments for acute weepy dermatitis. Chemists also need to consider the irritation or any sensitization potential to ensure that the topical application can be stable during storage and transport