1/2 pill twice daily.

He was given capstar for dogs weighing 51-100 lbs and the zeniquin which I have not started yet as I want to see if he vomitted which he did today and it's day 5. It was yellow so he must be hungry. I will give him a week to see if he continue and if not then will give the zeniquin. The bottle said to give one 50mg tablet a day so is ok if I give him half a tablet 2x a day?

Your dog would, I estimate, be about 40lbs or more at 10mos old, thus the dosing your vet prescribed is half of what the lowest amount tested/described in the article you referenced. All drug companies will test much higher than they probably will recommend for usage (and what will be cidal to the bacteria they are focusing on) to establish "safety margins". I see this on almost every drug listed in my Veterinary Drug Handbook, which is the standard reference in vet offices.

Again, if you are too nervous to use this, then speak with the vet again, but I didn't see anything that would send red flags to me personally...and its always valuable to know the signs of a possible negative reaction to any medication, so certainly the article is very valuable reading.

it somehow affects the articular cartilage which causes lameness. My dog is being prescribed 50mg once a day and will probably be on for 2-3 months.

Below is the study article, full article herehttp://www.drugs.com/vet/zeniquin.html

"The toxicity of marbofloxacin was assessed in 12- to 14-month-old beagle dogs administered marbofloxacin at 2.5, 7.5 and 12.5 mg/lb/day for 42 days. Vomiting, reddened skin (usually involving the ears) and reddened mucous membranes were occasionally observed in all groups, including controls, but were noted most frequently in the 12.5 mg/lb group. Decreased food consumption and weight loss were significant in the 7.5 mg/lb and 12.5 mg/lb groups. No clinical lameness was noted in any of the treated animals. Minimal to slight lesions in the articular cartilage were observed in 1/8 placebo-treated animals and in 3/8 animals given 12.5 mg marbofloxacin/lb. Macroscopically, these lesions were vesicles, raised areas, or depressed, light-colored areas. Microscopically, these lesions were characterized by the presence of one or more of the following: fissuring, erosion, chondrocyte proliferation, fibrillation, or vertical splitting of the articular cartilage. These cartilage lesions in treated dogs were similar to those in control dogs, and were not typical of those produced by fluoroquinolones. In addition to the above pathologic alterations, red areas of articular cartilage were noted macroscopically in 0/8 placebo-treated dogs and in 2/8 dogs from each of the three marbofloxacin-treated groups. These areas usually correlated microscopically with areas of vascularity of the articular surface, but could not be confirmed microscopically in all animals. They consisted of large blood vessels in mature fibrous connective tissue, with no indication of active vascularization due to drug-induced damage. They were considered most likely to be developmental anomalies or normal variations of the joint surface and were not considered to be related to drug treatment.
Marbofloxacin was administered to 12- to 14-month-old beagle dogs at a dosage of 25 mg/lb/day for 12 days. Decreased food consumption, vomiting, dehydration, excessive salivation, tremors, reddened skin, facial swelling, decreased activity and weight loss were seen in treated dogs. No clinical lameness was noted. As in the 42 day study, grossly visible, focal, red areas of articular cartilage were seen. These findings were noted in 2/6 placebo-treated dogs and in 4/6 marbofloxacintreated dogs. The foci were areas of fibrocartilage with prominent vascularization or increased vascularization of subchondral bone. Due to the appearance microscopically and macroscopically, these red foci were described as likely to be developmental anomalies or normal variations in articular cartilage."

your vet knows the age of your pet, most bulldogs reach their full adult height (and general weight) by 9-10mos old but they continue to "fill in" both in width and depth of chest, skull maturity until about two years of age. So based on averages and the fact that your vet recommended this (you didn't just pick it off your shelf at home) I would say its okay to use. You might want to post a question: Has anyone had negatives effects from Zeniquin??