Is a strong warming or burning feeling on the skin normal, and what precautions should I take?

Yes, a clear feeling of warmth on the skin is the intended effect. Formic acid works as a counter-irritant: it stimulates skin receptors on the surface, which the body perceives as heat. Some mild reddening of the skin in the applied area can accompany this and is a normal part of how warming rubs of this type work. The intensity varies from person to person — skin sensitivity, the amount applied and even how vigorously you massage all influence how strong it feels.

Because the formula contains formic acid and viper venom extract, do a patch test before first use: apply a small amount to a limited area such as the inner forearm, wait several hours (ideally 24), and check for any unusual reaction. If the warmth turns into genuine burning, itching, swelling or persistent redness, wash the product off with lukewarm water and mild soap and stop using it.

• Use on external, intact skin only — never on broken, freshly shaved or irritated skin, where the actives would feel much harsher.
• Keep it away from the eyes, mouth and other mucous membranes.
• Avoid applying immediately before a hot shower, sauna or heating pad, as heat can intensify the sensation considerably.

If you are pregnant or breastfeeding, have a chronic condition or very reactive skin, consult a specialist before use.