Penciclovir is a guanosine analogue antiviral drug used for the treatment of various herpesvirus infections. It is a nucleoside analogue which exhibits low toxicity and good selectivity. Because penciclovir is absorbed poorly when given orally (by mouth) it is more often used as a topical treatment. It is the active ingredient in the cold sore medications Denavir (NDC 0135-0315-52), Vectavir and Fenivir. Famciclovir is a prodrug of penciclovir with improved oral bioavailability. Penciclovir was approved for medical use in 1996. Medical use
In herpes labialis, the duration of healing, pain and detectable virus is reduced by up to one day, compared with the total duration of 2–3 weeks of disease presentation. Mechanism of action
Penciclovir is inactive in its initial form. Within a virally infected cell a viral thymidine kinase adds a phosphate group to the penciclovir molecule; this is the rate-limiting step in the activation of penciclovir. Cellular (human) kinases then add two more phosphate groups, producing the active penciclovir triphosphate. This activated form inhibits viral DNA polymerase, thus impairing the ability of the virus to replicate within the cell. The selectivity of penciclovir may be attributed to two factors. First, cellular thymidine kinases phosphorylate the parent form significantly less rapidly than does the viral thymidine kinase, so the active triphosphate is present at much higher concentrations in virally infected cells than in uninfected cells. Second, the activated drug binds to viral DNA polymerase with a much higher affinity than to human DNA polymerases. As a result, penciclovir exhibits negligible cytotoxicity to healthy cells.
A blood blister is a type of blister that forms when subdermal tissues and blood vessels are damaged without piercing the skin. It consists of a pool of lymph, blood and other body fluids trapped beneath the skin. If punctured, it suppurates a dark  fluid. Sometimes the fluids are cut off from the rest of the body and dry up, leaving behind dead cell material inside the blister with a texture like putty. Some blood blisters can be extremely painful due to bruising where the blister occurred. There are also blood blister-like aneurysms as these are known to be located in the supraclinoid internal carotid artery and have been recognized as having unique pathological and clinical features. Causes
Blood blisters are commonly caused by accidents in which the skin is pinched by a tool, mechanism, or heavy weight without protective equipment. Blood blisters can also arise from forcible human contact, including grappling. Blood blisters also may occur with friction caused by constant rubbing of skin against a surface. Ill-fitting shoes that rub on the skin can cause the blood vessels in the skin to break and form a blood clot under the skin, resulting in a blood blister. Certain sports activities that require repeated movement and rubbing of the skin against equipment may also cause this. So baseball pitchers, rowers, and drummers often contract blood blisters on the fingers and palms. They also form as a result of frostbite.