Sublingual Vitamin B12 supplements are generally considered safe and are not known to cause harm even at high doses[^1][^2]. It is unusual for Vitamin B12 itself to cause stomach issues. Sublingual B12 is absorbed under the tongue and bypasses the digestive system to some extent, which typically reduces gastrointestinal side effects[^3][^4].

However, there are a few considerations if you're experiencing stomach discomfort:

1. Inactive Ingredients: Sometimes, the inactive ingredients or fillers in supplements can irritate the stomach.

2. Overdose: Although B12 is not known to be toxic, consuming very high doses can lead to mild digestive discomfort or imbalance in sensitive individuals.

3. Allergic Reaction: It's possible to have a sensitivity or allergy to one of the components in the supplement formulation.

4. Medication Interactions: If you are taking other medications, they could interact and affect your vitamin B12 levels or absorption[^5].

If these symptoms persist, it might be helpful to consult with a healthcare professional to ensure there aren't other underlying causes, such as stomach sensitivity, an unrelated gastrointestinal issue, or interactions with other medications you might be taking.