There are many sites on the web that claim that lemon juice is effective in removing warts (see this Google search).
Information varies. Some sites say that application several times a day is enough, and results are fairly rapid. Others suggest overnight soaking is necessary and the result could take weeks.
Does the application of lemon juice to a wart result in the wart's disappearance at all?
According to Evaluation of the efficacy of 50% citric acid solution in plane wart treatment
After six weeks 64.4% of the lesions in citric acid treated group disappeared versus 53.7% of the lesions in tretinoin treated group. This difference was significant ( P value Conclusion: On the basis of this study, the treatment of plane warts by 50% citric acid is strongly suggested. This modality is superior to tretinoin lotion due to higher efficacy and low incidence of side-effects and lower cost.
In each patient, lesions of one side of the body were treated by application of 50% citric acid aqueous solution and the other side by 0.05% tretinoin lotion, twice daily.
However, lemons are only about 4% citric acid.
TL;DR: No, lemon juice has not been shown to work. Other treatments have been shown to be more efficacious and safe, making them better candidates for initial treatment of (non-genital) warts.
Wart treatments have had a lot of research - much of it poor - and a number of meta-analyses have been written to evaluate the information. We can see the medical literature progressing over the past decade in this area.
In 2002, Local treatments for cutaneous warts: systematic review complained that the 50 included trials "provided generally weak evidence".
High quality research on the efficacy of various local treatments for warts is lacking
Evidence, which is generally of a poor quality, shows a beneficial effect of topical salicylic acid and contact immunotherapy with dinitrochlorobenzene
Little evidence exists for the efficacy of cryotherapy and no consistent evidence for the efficacy of all the other treatments reviewed
It did not include citric acid.
In 2005, Cutaneous Warts: An Evidence-Based Approach to Therapy looked at the existing research, but did NOT include citric acid as a potential treatment. It recommended topical salicylic acid and physician-administered cryotherapy, with intralesional immunotherapy for large or recalcitrant (non-genital) warts.
In 2012, Treatment of cutaneous warts: an evidence-based review recommended salicylic acid, silver nitrate and glutaraldehyde first or for mild cases; cryotherapy if that doesn't work, and then if none of the above work, "a variety of alternative therapeutic options (topical, intralesional, systemic, and physical destruction) that are generally off-label (not US FDA approved), and whose use is limited by drawbacks or adverse effects."
I haven't been able to read the entire paper, but they do cite Evaluation of the efficacy of 50% citric acid solution in plane wart treatment. (See @JamesChristopher's answer for more.) Thus it appears they did consider citric acid as one of these alternative topical treatments.
However, they don't hold much hope for this third line of attack:
From pooled evidence-based medicine data, it is possible to conclude that significantly higher remission rates may be expected only with cryotherapy and salicylic acid used in combination.
In 2013, Topical Treatments for Cutaneous Wart: An update, explained:
Little has changed in the field over the past 8 years.
It did not explicitly look at citric acid, but looked at other topical acids.
It recommended cryotherapy and salicylic acid, and complained:
Good quality data for most of the other treatments are still lacking.
but admitted that topical immunotherapy with dinitrochlorobenzene has some evidence to support it.
It was fairly generous about other treatments:
Treatment decisions should be made on a case-by-case basis, according to evidence-based medicine criteria, but also considering patient preference and physician experience. So far, the use of nonconventional approaches in selected cases of relapsing and refractory warts seems justified, although scientific evidence is still low.
As James Christopher explains, even if 50% citric acid was effective, lemon juice holds a much lower concentration. Without proper studies of lemon juice, it is an inappropriate treatment - especially when better treatments have proven to work.
