This is the supplement here people use:

I copy the label:
CEBION 0.1 g / 1 ml drops 30ml oral drops contain vitamin C (ascorbic acid) for babies
Ingredients:
1 ml (approx. 20 drops) contains 100 mg of ascorbic acid (vitamin C)
Action: Vitamin C increases iron absorption, inhibit the aging process - is an antioxidant, is irreplaceable in the process of regeneration of the body during weakening and colds - supports immune processes. She participates in the prevention of diseases caused by deficiency of vitamin C - mainly scurvy and its initial symptoms and Moeller-Barlow disease. Vitamin C is also involved in the synthesis of collagen, and at the same time maintain good condition of connective tissue and bone. The ingress of blood vessels. It is recommended, especially symptoms of deficiency manifesting: vascular fragility, bleeding, delayed wound healing, inflammation of the gums, the changes in the structure of bones and teeth. Vitamin C is also administered prophylactically to babies fed artificially.
Usage: Infants given 5-8 drops per day (25-40 mg of vitamin C); young children - 10 drops per day (50 mg of vitamin C). Therapeutic - The above dosages should be applied two times a day.Take neat or in diluted form, eg. added to milk, juice or slurry.
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each drop contains 6mg of vit C. The daily amount for a guinea pig is 30-40mg/kg. As my piggies weigh 600-700g and eat peppers and fresh grass I give each piggie only 2 drops mixed with 3ml of water using a little syringe. In Italy Cebion
10ml costs 4-6€ depending on the chemist's (buying it online costs less) and for two piggies (2 drops) it lasts more than one month.