Rhino dehorning is increasingly seen as a method of halting poaching in vulnerable black rhino populations. However, mathematical models suggest that, under current costs and prices, dehorning must be done annually if poaching is to be made unprofitable. As dehorning carries a risk of rhino mortality, it is unsustainable as an anti-poaching measure. A profit-maximizing manager will dehorn at less frequent, but still unsustainable, intervals. Sustainable dehorning produces near-optimal profits but will not deter poachers.