Fleas are a result of toxins
They are 'canaries in the coal mine', alerting you to problems in the making in your cat. Fleas consume the toxins, so are doing a valuable job.
Toxins come from commercial cat food and from veterinary medications. This compounds over the years, causing major health issues.
Rather than use the highly toxic and neurotoxic veterinary solutions, it's healthier to get to the root cause of the problem.
The best way to control fleas is to feed your cat a healthy, quality raw meat and bones diet and to use holistic health care, such as homeopathy.
However, in the early stages of the dietary conversion, fleas may be in abundant as the cat de-toxes - at last! Diatomacious earth (food grade NOT garden grade) VERY lightly rubbed into the fur can kill fleas. As can brewers yeast. Just be careful to use them sparingly so the cat doesn't inhale it. Both are safe and effective.
This homeopathic complex is made from several homeopathic remedies in a low potency that are useful in resolving an infestation of fleas in a cat. The vial size is 15ml. It comes with a dropper for ease of use. There is enough that will probably last the life of your cat, as long as you are feeding them appropriately.
The dose is two drops on food or in the water, daily until the issue is resolved. Then stop.