Nowadays, there are several types of artificial sweetener. They do not include calories or are hard to be absorbed in the human intestines. Therefore, they seemed not to be harmful despite they taste sweet.
However, artificial sweeteners are often criticized recently. Some researchers claim they can raise blood sugar level in spite of they lack energy. Also, insulin is secreted when the brain feels sweet even if there is no sugar to be absorbed from the blood into the tissue, according to another researcher.
Are sweeteners risky for us as well as white sugar? Fortunately, or unfortunately, there is no evidence to support it.
A recent meta-analysis study revealed that combination of recent studies suggested the stability of blood sugar level in taking artificial sweeteners.
Nature: Glycemic impact of non-nutritive sweeteners: a systematic review and meta-analysis of randomized controlled trials
The collected articles in this study were randomized-controlled trials (RCT). Therefore, the result of meta-analysis is relatively trustworthy. As far as I read the article, there are no critical flaws in this article.
It does not mean, however, sweeteners are not harmful at all. There are several possibilities of harm caused by artificial substances. We need not be afraid of them too much. But I recommend you not to depend on artificial sweeteners in your diet.