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Famvir Side Effects

Famvir is a prescription medication designed to treat herpes. All prescription medications have side effects. This article explores the side effects of Famvir.
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Famvir (drug name famciclovir)is a popular medication for preventing replication of viruses in the herpes family.

Famvir is generally prescribed to reduce the duration and severity of Shingles (also known as herpes zoster), herpes simplex type 1 (traditionally oral herpes), and herpes simplex type 2 (traditionally genital herpes). It may also be prescribed for off label uses, including ocular herpes and chicken pox (both within the herpes family of viruses.

Before taking any medication, it is important to fully understand the risks. Below is information regarding any potential side effects of Famvir.

Are There Any Famvir Side Effects?

All medications have risks. Yet compared to other types of medicines, Famvir is very well tolerated. Common side effects of Famvir include:

  • Headache
  • Diarrhea
  • Nausea
  • Gas
  • Vomiting

Headache, diarrhea, and stomach discomfort are the most common Famvir side effects, occurring in more than 10% of the population.

It should be noted that placebo (sugar pill), which has no effect on the body, may also lead to headache, nausea, and diarrhea. So while Famvir may cause these side effects, they are comparable to the side effects of taking a placebo.

According to a study published by the National Institute of Health, only when treating recurrent genital herpes did subjects report a risk of headache greater than 8% risk of headache when compared to placebo. All other side effects (nausea, diarrhea, etc.) and all other treatments (genital herpes suppression, herpes zoster treatment, and herpes labialis) did not show any significant differences Famvir and placebo.

Are There Any Dangerous Famvir Side Effects?

Currently there are no known severe side effects of Famvir. It should be noted, however, that Famvir is still a relatively new drug, so long term side effects may not be well known.

The most serious risk of Famvir is the risk of an allergic reaction. Doctors note that an allergic reaction to Famvir may cause a host of potentially dangerous symptoms including, but not limited to:

  • Wheezing
  • Difficulty Breathing
  • Hives
  • Swelling in the Lips, Throat, Tongue
  • Muscle Pain

Those that experience an allergic reaction to Famvir are also at risk for any diseases related to severe allergic reaction, such as Stevens-Johnson Syndrome, decreased white blood cell count, Erythema Multiforme and more. These severe reactions are exceedingly rare, and not limited to Famvir but rather possible complications of severe medicinal allergic reactions.

Contact your doctor immediately if you show any signs or symptoms of an allergic reaction, whether mild or severe.

Can Famvir Affect Sexual Health?

According to a study of Famvir in rats, some male rats experienced testicular toxicity at high doses – higher than recommended for any reasonable treatment. This lead to some concern that Famvir could affect sexual health, especially in men.

However, studies of human sperm count have indicated that men do not appear to be affected by reasonable doses of Famvir. Women also appear to be at no risk, although nursing and expectant mothers are advised against taking Famvir unless the benefits to the mother are greater than the risk to the fetus or child.

What is the Risk of Famvir Overdose?

Few to no studies have been completed about the effects of Famvir when overdosed. It is recommended that if you believe you've overdosed you contact a doctor immediately. Overdose of other antivirals, such as Valtrex, may cause dangerous side effects including:

  • Kidney Failure
  • Seizures
  • Coma

It is not known whether Famvir would cause the same symptoms in overdose form, but it is believed that an overdose would be similar. Famvir, like all drugs, should be taken only as directed by your doctor. Contact your doctor or call 911 if you believe you may have overdosed on Famvir.

Complications That May Lead to Famvir Side Effects

Famvir is primarily eliminated through the kidneys, so those with impaired kidney function should be expected to take a lower dose than those with healthy kidney function.

Similarly, those with liver damage may be advised against taking Famvir. Studies did not show any significant differences between those with liver damage and those without on Famvir side effects or dangers. However, few studies in this area are known, so the FDA cannot yet recommend Famvir in liver compromised patients.

There also did not appear to be any significant differences in side effects of Famvir between races, genders, or age groups. Famvir has not been approved for youth populations, but no known Famvir side effects are believed to occur.

Medications that May Interact with Famvir

Based on initial research published by NIH, there are few to no drug interactions with Famvir. Studies have yet to identify any drug that may cause any additional Famvir side effects.

Still, it is not reasonable or feasible for researchers to identify and contrast every type of known drug to Famvir, so drug interactions may still occur. That is why it is always important to discuss any other drugs you are taking with your doctor, and call 9-1-1 if you feel any side effects that resemble a drug interaction.

Can Pencilclovir Have Side Effects?

Famvir is also a pro-drug. Pro-drugs are drugs that are metabolized to turn into other drugs. In the case of famciclovir, it is metabolized to into Penciclovir. The reason a pro-drug is necessary is because penciclovir becomes weaker once it enters the body. By placing it in famciclovir form, the drug survives breakdown in the stomach and has a more powerful effect on the herpes virus.

Conceivably, this breakdown could lead to additional Famvir side effects, either as a reaction to the initial famciclovir drug or as a reaction to Penciclovir. As of this writing, however, there are no known additional side effects that occur as a result of this transformation. 

Overall Safety of Famvir

Like all medications, Famvir has risks, and it is possible that not all of the side effects of Famvir are known. Famvir is a more recent drug that, while effective, has not been studied with every individual in every potential harmful scenario. It is possible for drug interactions, side effects, and long term damage to all be caused by Famvir over the course of your treatment.

Nevertheless, studies do indicate that Famvir has few side effects and is a worthwhile treatment to consider for oral herpes, genital herpes, and shingles. 



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