Famciclovir is a medication commonly used to treat viral infections, such as herpes simplex and varicella-zoster virus infections [1][2]. However, certain populations should exercise caution when using this medication due to potential increased risks or altered effectiveness.

Firstly, individuals with impaired renal function should be cautious when using famciclovir, as the drug is primarily excreted by the kidneys [1][2]. Dosage adjustments may be necessary for patients with renal impairment to prevent the accumulation of the drug in the body, which could lead to toxicity [2].

Secondly, elderly patients may also require dosage adjustments due to age-related decreases in renal function [2]. Moreover, elderly patients are at an increased risk of adverse reactions due to polypharmacy and altered drug metabolism [2].

Thirdly, pregnant women should use famciclovir with caution, as the safety of the drug during pregnancy has not been established [1][2]. The FDA has classified famciclovir as a pregnancy category B drug, which means that animal studies have not shown a risk to the fetus, but there are no well-controlled studies in humans [3].

Lastly, breastfeeding women should also use famciclovir with caution, as the drug is excreted in breast milk [1][2]. The American Academy of Pediatrics considers famciclovir compatible with breastfeeding, but caution is advised [2].

In summary, individuals with impaired renal function, the elderly, pregnant women, and breastfeeding women should exercise caution when using famciclovir due to potential increased risks or altered effectiveness. Dosage adjustments may be necessary for some of these populations [1][2][3].

Sources:
[1] DrugPatentWatch. Famciclovir. <https://www.drugpatentwatch.com/p/tradename/FAMCICLOVIR>
[2] Facts and Comparisons. Famciclovir. <https://jamanetwork.com/journals/jama/fullarticle/186468>
[3] FDA. Famvir (famciclovir) tablets. <https://www.accessdata.fda.gov/drugsatfda_docs/label/2011/020363s037lbl.pdf>