There are also anecdotal accounts from West Africa of neem-leaf teas possibly causing kidney damage when taken over a long period.
The ultimate significance of these preliminary studies is unclear. The materials used may well have been contaminated—perhaps by fungal toxins. Other trials have found no toxicity problem. For instance, in a toxicological study in Germany, neem oil obtained from clean, fungus-free seed kernels showed no oral toxicity in rats. The dosage tested was 5,000 mg per kg body weight.8
Certainly, no hazard has been observed when neem has been used in topical treatments (on skin complaints, for example) or in dental uses—which together make up by far the major medical applications. Nor is there any evidence that using the seed-kernel extracts as pesticides is hazardous to health.
Whether neem oil is safe for possible use as an intravaginal spermicide is also unclear. However, in this use there are perhaps even greater uncertainties. Contraception is a topic of such sensitivity—personal, political, scientific, and religious—that here, too, its future role is uncertain.
Its use as a spermatocide—the male contraceptive mentioned earlier—is so uncertain that it will likely take decades of research to develop even if no safety hazards are found along the way.
The number and complexity of the compounds in neem extracts will always preclude the economic synthesis of the full mixture. On the other hand, individual compounds may prove suitable for synthesis. There is, therefore, the possibility that if neem opens up a new generation of pesticides, synthetic mimics may capture some of the more lucrative "top-end" markets.