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I have used Neem oil (Fertilome's 3-in-1) on some tomato plants twice since I began getting into vegetable gardening. The thing is that I have heard/read, on one hand, that it is not toxic and quite "natural", but on the other hand, I have heard/read people claiming that it is toxic.

Most online searches I have done show a lot of subjective accounts that just contribute to the ambiguity.

I am looking for answers that provide information that is as unambiguous as possible.

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    npic.orst.edu/factsheets/neemgen.html - Basically in India it gets used in cosmetics. Like Tung Oil, it is a nut oil, so people with nut allergies should avoid it.
    – Fiasco Labs
    Commented Nov 17, 2013 at 21:09
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    "natural" and "toxic" are not mutually exclusive either. As well as caveats (cf. Fiasco Labs' answer), something like ricin or botulinum toxin are both very toxic and very natural!
    – winwaed
    Commented Nov 19, 2013 at 15:11

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This paper reviews toxicological data from human and animal studies regarding neem derived pesticides. I don't have access to the full paper but in the abstract it says that

The non-aqueous extracts appear to be the most toxic neem-based products, with an estimated safe dose (ESD) of 0.002 and 12.5 microg/kg bw/day. Less toxic are the unprocessed materials seed oil and the aqueous extracts (ESD 0.26 and 0.3 mg/kg bw/day, 2 microl/kg bw/day respectively). Most of the pure compounds show a relatively low toxicity (ESD azadirachtin 15 mg/kg bw/day).

and

This leads to the conclusion that, if applied with care, use of neem derived pesticides as an insecticide should not be discouraged.

However, in very large doses neem oil can be toxic. According to this recent paper the LD50 value of neem oil is 31.95g/kg but the abstract doesn't say if this is for humans or a specific type of animal. However, since this old paper says that

The 24-h LD50 was established as 14 ml/kg in rats and 24 ml/kg in rabbits

so I am assuming that the first LD50 is for humans.

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    Wow, sorry for not voting your answer sooner. I realized that I had email notifications off. Plus I had to postpone my garden activities for a few months. Anyway, your answer is precisely was I was looking.
    – YOMorales
    Commented Apr 7, 2014 at 3:02
  • And @kevinsky, thanks for giving him a bounty. :)
    – YOMorales
    Commented Apr 7, 2014 at 3:05
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The main insecticide component of this oil is Azadirachtin. It said to be almost non-toxic to mammals.

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    The Wikipedia entry does not reference any academic studies that support the claims of low toxicity to mammals. Can you expand on your answer?
    – kevinskio ♦
    Commented Nov 26, 2013 at 17:07

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