Seasonal allergic rhinitis, otherwise known as "hay fever" is an uncomfortable, but benign, condition. In half the patients, symptoms become less pronounced with age. When the discomfort persists despite symptomatic treatment, desensitisation is sometimes offered, if the pollen(s) responsible has/have been identified. In these patients, what is the harm-benefit balance for desensitisation by the subcutaneous or by the sublingual route? We carried out a review of the available evidence. According to systematic reviews of randomised trials, desensitisation to pollens by the subcutaneous route has demonstrable, but weak, efficacy in reducing symptoms of allergic rhinitis and reducing the consumption of medicines. Subcutaneous desensitisation carries a risk of serious hypersensitivity reactions. Under trial conditions, more than 1 of the patients treated by subcutaneous desensitisation received an injection of adrenaline following a severe reaction. Mortality due to subcutaneous desensitisation is around 1 death per 50 000 patients treated. Factors predisposing to severe reactions include poorly controlled asthma, rapid escalation of allergen doses, a history of systemic allergic reactions following a previous injection, and desensitisation during the pollen season. Desensitisation by the sublingual route seems to save, on average, a few days of symptomatic treatment in each pollen season. According to one small-scale randomised trial, and indirect comparisons, it does not seem to be more effective than subcutaneous desensitisation in reducing the symptoms of seasonal allergic rhinitis and reducing medication use. Sublingual desensitisation is more convenient than subcutaneous desensitisation, but it too carries a risk of serious adverse effects and with a lower level of monitoring by healthcare professionals. In practice, the poor efficacy of subcutaneous desensitisation does not justify running the repeated risk of serious, sometimes fatal, hypersensitivity reactions. As of 2018, the harm-benefit balance of sublingual desensitisation remains uncertain.
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