Black seeds have been eaten as a spice for centuries, they have been found to be safe even in quite high doses. 
Pure black seed oil is the pressed lipid component of the Nigella sativa plant seed. Black seed oil is much more concentrated than the equivalent volume of seed. Around seven spoons of black seed are required to produce one spoon of black seed oil.
What are the side effects of black seed oil?
There is a wide margin of safety for therapeutic doses of black seed oil.  Like any natural supplement you should exercise caution when first taking black seed oil.
Some individuals will simply not get along with black seed oil (very rare) and some may even be allergic to it (even rarer). With any new supplement start with small amounts and increase gradually.
Effects of black seed oil to be aware of: –
Understanding certain effects black seed oil can have on different bodily functions is important before taking.
1. Hypotensive. Black seed oil can reduce you blood pressure, make sure you don’t stand up too quickly, keep hydrated and ask you doctor. to monitor you blood pressure whilst on black seed oil. You may need to reduce or come off blood pressure medications under supervision of your doctor.
2. Hypoglycaemic. Black seed oil can regulate blood sugar so may alleviate the need for diabetic medications. Make sure you eat regularly whilst taking the black seed oil (best taken on an empty stomach 30 mins before a meal). If you are diabetic monitor you sugars and tell your doctor. You may need to reduce or come off diabetic medications under supervision of your doctor.
3. Sedative. After 30 minutes of taking black seed oil you may feel lightheaded and drowsy. This will depend on your general state of alertness. This state is useful for the evening dose in helping you nod off to sleep. Until you know how black seed oil affects you don’t drive or operate heavy machinery soon after taking it.
4. Blood thinning. Black seed oil can slow down clotting. If you take any anti clotting medication speak to your doctor. so a blood clotting profile test can be done whilst on black seed oil.
Black seed oil should not be taken by pregnant women.
- Ali BH, Blunden G. Pharmacological and toxicological properties of Nigella sativa. Phytother Res. 2003 Apr;17(4):299-305.
- Zaoui A, Cherrah Y, Mahassini N, et al. Acute and chronic toxicity of Nigella sativa fixed oil. Phytomedicine. 2002 Jan;9(1):69-74.