Cold Sore Inhibitor Side Effects

Cold sores result from infection with either one of two types of the herpes simplex virus, usually herpes simplex-1. Since antiviral drugs used to treat cold sores work by inhibiting the replication of the herpes simplex virus rather than killing or curing it, they are often referred to as cold sore inhibitors. In the 2008 edition of "Basic and Clinical Pharmacology," University of California-San Francisco Professor Sharon Safrin explains that, while cold sore inhibitors are generally considered safe and well-tolerated, as with any drug, some people experience side effects.

Headaches

Cold sore inhibitors appear to modestly increase the risk of headaches in both adults and children. In clinical trials for the cold sore inhibitor, acyclovir, manufacturer GlaxoSmithKline found that just 2.2 percent of recipients reported headaches. In clinical trials for the related drug, valacyclovir, GlaxoSmithKline reported that the incidence of headaches was higher--about 4 percent. This risk, however, may have been influenced by the dose, since participants in the study received high, one-time doses of valacyclovir, compared to lower doses over a seven-day course in the acyclovir study. Headaches can accompany a cold sore outbreak, even among patients who don't take cold sore inhibitors. MayoClinic.com recommends over-the-counter acetaminophen or ibuprofen to fight headache pain.

Gastrointestinal Problems

Cold sore inhibitors appear to modestly increase the risk of nausea, vomiting, diarrhea and flatulence in adults. In clinical studies for the cold sore inhibitor famciclovir, the manufacturer, Novartis Pharmaceuticals, reported that the incidence of nausea, diarrhea, vomiting and flatulence in treated patients exceeded the incidence in untreated patients by just 2.9 percent for diarrhea, 1.4 percent for vomiting, and less than 1 percent each for nausea and flatulence. The incidence of abdominal pain was actually higher in untreated patients compared to treated patients. Taking the medication with food does not influence the risk of gastrointestinal problems.

Skin Reactions

Cold sore inhibitors modestly increase the risk of skin reactions in people of all ages. Safrin reports that about 2 percent of people who use topical cold sore inhibitors such as penciclovir or docosanol experience skin reactions such as acne, dryness, redness, itching or burning or other kinds of itching. Interestingly, an April 2007 study published in the "Journal of Drugs in Dermatology" by University of Washington dermatologist Stanley C. Gilbert, M.D. also found an increase in the incidence of chapped lips among patients who took oral valacyclovir. As with other kinds of side effects, side effects in Gilbert's study were dose dependent. Patients who experience troublesome side effects of cold sore inhibitors should talk to their doctors about lower dose treatment regimens.