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Both acyclovir and valacyclovir are medications that are used to treat the viruses known as herpes simplex, varicella zoster, and herpes zoster. These viruses are responsible for certain diseases like chickenpox and shingles, and the drugs can help prevent them from replicating in the body. Despite the overt similarities between these drugs, they have important differences between them that may determine how and when they are used in a clinical setting.
A major difference between acyclovir and valacyclovir pertains to their bioavailability, or how efficiently they are absorbed into the bloodstream after ingestion. Valacyclovir tends to be absorbed more slowly than acyclovir, regardless of whether it is administered as a topical solution, by mouth, or via intravenous (IV) means. These two drugs are also broken down by the body at different rates. Acyclovir is metabolized my quickly, meaning that it must be taken more frequently than valacyclovir. The latter drug may be a better option for patients taking large numbers of medications, because they do not have to remember to take this medication as often.
How much is taken also differs. Orally, acyclovir is usually taken every four hours, with dosages of 200 milligrams (mg) to 800 mg. The longer duration of action for valacyclovir means that it is only taken every eight hours, with a 1,000 mg dosage each time. These medications should typically be taken for at least a week, or as long as directed by a medical professional, because an infection may still be present after symptoms disappear. Discontinuing these medications too early can result in an infection reappearing, and the rapid mutation rates of viruses may allow them to become resistant to these drugs if they are not completely destroyed during the first course of treatment.
Drug interactions for acyclovir and valacyclovir are generally quite similar, although there are some differences. Phenytoin and valproic acid are more likely to have their effectiveness reduced by acyclovir, for example. Valacyclovir is more likely to be affected by cimetidine. This interaction slows the metabolism of the antiviral drug, making it exert its effects longer and leading to a higher chance of side effects.
The side effect profiles for both medications are also quite similar, with slight variations. Diarrhea is slightly less common among people taking valacyclovir. Both antivirals tend to cause nausea and headaches in people who take them, however.