Yes it really is poisonous. Got this on the CSIRO website:

Are green potatoes safe to eat?
Green potatoes may cause food poisoning and since some of the symptoms are similar to gastroenteritis it is possible that some undiagnosed cases of gastroenteritis have been caused by eating green potatoes.

Human and livestock deaths have been recorded as a result of the consumption of greened or damaged potatoes with very high glycoalkaloid levels. It should be noted that glycoalkaloids are not destroyed by cooking processes, even by frying in hot oil. Consequently potatoes with pronounced greening or with signs of damage should not be eaten.

It is advisable that green or damaged potatoes are avoided by pregnant women or women who are likely to become pregnant, as there is some evidence of possible foetal damage or loss of the foetus from glycoalkaloid poisoning in animals.

...
Conclusions
Not every potato with traces of greening will contain sufficient levels of glycoalkaloids to pose a threat to health. However because of the possibility that green potatoes may produce food poisoning, they should be discarded, as should physically damaged potatoes and those with any signs of rotting.

Consumers should avoid buying any potatoes that show signs of greening or damage and should carefully remove any sprouts before cooking. However it should be remembered that healthy potatoes do not pose any health risk at all and are an excellent source of nutrients.
It is helpful to realise that the poisonous compound tastes quite bitter, so if your potato has that real bitter flavour