About Stevia : I don't know for sure what this product is so I will assume it is just Stevia rebaudiana extract. There has been a huge controversy over the safety of the product, but serious studies (WHO - Safety Evaluation of Certain Food Additives: Steviol Glycosides) tend to prove there's no significant effect on health (not a mutagen) in "consumption" doses.
The Committee concluded that stevioside and rebaudioside A are not genotoxic
in vitro or in vivo and that the genotoxicity of steviol and some of its oxidative
derivatives in vitro is not expressed in vivo.
The no-observed-effect level (NOEL)
for stevioside was 970mg/kgbw per day in a long-term study evaluated by the
It should also be noted that it is not an "inert" compound has apparently some effects on humans. Please ask a doctor if you think you might be affected by the Stevia extract you use.
The Committee noted that stevioside has shown some evidence of pharmacological
effects in patients with hypertension or with type-2 diabetes [..]. The evidence available at present was inadequate to
assess whether these pharmacological effects would also occur at lower levels of
dietary exposure, which could lead to adverse effects in some individuals (e.g.
those with hypotension or diabetes).
All depends of the flavor.
Hazelnut flavor is most likely to be filbertone but it could also be a mix of different other chemicals I'm not aware of. Filbertone is naturally present in hazelnut.
I am not aware of any study claiming that filbertone is harmful but also none that claimed it was totally safe. Normally it should be present in very small quantity.
Normally your stevia drops should contain information about its composition. Ask a someone competent if you want detailed information. It can be a doctor, a chemist or a pharmacist.