Nicotine booster 20mg/ml in a 10ml bottle from French brand Eliquid France, with a 50% VG / 50% PG base.
Add it to your flavor shots or DIY e-liquids to reach the nicotine strength you want. For example, one 10ml booster in a total of 60ml of liquid gives about 3.3mg/ml.
Warning: This product contains nicotine, which is a highly addictive substance. Keep out of reach of children.