It isn't that the drops will harm your pigs, it is that they do no good at all.
We warn against using them because they can make water taste pretty bad therefore pigs won't drink their water. If your pig is drinking, then he/she is fine.
Pet stores promote the use of the drops because it is money in their pocket. The truth is Vit C breaks down very quickly in water, so unless you pour out the water and replace it with new water and drops SEVERAL times a day, then your pig isn't getting the vital vit c that they need. Also, because he drops are diluted in to water, unless your pig is emptying out his bottle every day you don't have any way of knowing if they are even getting enough of the drops. (guinea pigs cannot make nor maintain their own levels of Vit C, and a vitamin C deficiency can make them very very ill, and kill them)
Guinea Pigs do not need vitamin suppliments if they are getting a proper diet. They should have fresh water, an unlimited supply of grass hay, and about 1/4th cup daily of quality pellets (nothing with seeds, nuts, corn, and colored bits in them---these things are all health and chocking hazards) as their base diet. They should acquire their vitamin C through a cup of vegetables a day. Romaine and leaf lettuces, bell peppers of all colors, fresh cilantro, and a baby carrot are all great sources of nutrition for them. There is a more detailed list of good veggies and fruits and a guideline of how often you can give each particular thing here:
http://www.guineapigcages.com/forum/...on-charts.html
For more information on guinea pig care, visit this link
Guinea Lynx :: Guinea Pig Care Guide
It is the best careguide to have at your finger tips.