There are a few potential side-effects to foam sclerotherapy you should be aware of, but you may not experience any of these:

• Bruising – Usually little to no bruising occurs. If you do experience bruising it will pass in a few weeks
• Pigmentation – Affects 20-30% of patients, especially those with fair skin and with large
  varicose veins near to the skin surface. This usually fades over a few months. Only permanent in rare cases
• Lumpiness – disappears over a few months, depending on the size of the varicose veins. Only permanent in rare cases
• Thrombophlebitis: A painful red lump may develop following treatment of especially large/lumpy varicose veins. Usually treated with anti-inflammatory medication (e.g. ibuprofen gel and/or tablets). Rarely the surgeon may carry out a further small procedure to remove clotting

Your surgeon will make sure you are fully aware of the risks and likelihood of them occurring.