I have a rx for a compound cream for my small fiber neuropathy which contains Ketoprofen/Lidocaine/Gabapentin/Amitriptyline. I'm curious how often this can be safely used. The rx says to use one or two times daily, but I get the impression from a brief conversation with the pharmacist and reading a little about it that you can use it pretty liberally. Does anyone know?

The question should be directed to the pharmacist who made it.

The cream itself is a special formulation called "transdermal" meaning it carries the drugs thru the skin easily and into the blood eventually. The amount absorbed will be proportional to the area applied. Usually these topicals are dispensed in small containers.

We used a transdermal gel, to dose our cat with cancer recently. She fought us for any oral administration, so I ground up her dose of prednisolone and mixed it into the transdermal gel I bought from the pharmacy, and we applied it to the tip of her inner ear.
This dose was all absorbed, as she went into remission.

So I would think the drugs in the cream would be well absorbed.

Drugs given orally you know are metabolized by the liver before they get to the rest of the body. This is called first pass. So oral drugs tend to be high in dose to account for the loss in concentration during first pass. When you inhale in the nose, or apply to the skin or under the tongue, there is no first pass, so the dose is much lower. Gabapentin is poorly orally absorbed, in the first place, and NOT metabolized so first pass is not there for it.

Amitriptyline has to be metabolized to an active form (metabolite) to even work and this is done by the liver. The lidocaine would exert most of its effects where applied, and some then into the circulation.

If you have the % of drug content...you can figure some of it out yourself. Say10% of one of the drugs, out of the total weight of the jar... would give the total amount of drug present in the jar. Say, 120 gm jar, 10% ketoprofen...would be 12 grams of it in the whole jar. Divide that by the # of doses used would give the amount of drug in each application. Ketoprofen in topical products, typically is 2.5% in the commercial products. Ketoprofen orally comes in 50, 75, 100, 150, and 200 mg capsules. Amitriptyline is typically 10mg orally to start.

The problem with estimating how much gets into your blood stream of each, really has not been published. I can't find it.
So these creams are made based on trial and error. The ketoprofen and amitriptyline are the most potent drugs in there.
And long term use of ketoprofen and diclofenac (both lipophilic NSAIDs) have shown some liver toxicity over time when applied topically.
This link gives warnings on diclofenac (which is more common in commerically made gels for arthritis relief)...
http://www.rxlist.com/voltaren-gel-d...de-effects.htm

So I don't think you should slather this on... but you can use it in
targeted areas that are small, a bit more often. I'd keep a log as to duration of action and size of the area applied. Any side effects should be logged too. Then when you finish the jar, count the days, and see how much per day you used. Then work back using the % to see how many milligrams you used that day, and then compare it to the oral dose. If it is HIGHER than a daily oral dose, I would be concerned. The ketoprofen and amitriptyline are the most critical and potentially toxic over time compared to the other two ingredients.

For example, the Kirkman cream of magnesium comes with a little scoop that measures 1 gram. They formulate their product commercially to have 100mg of elemental magnesium/scoop. This product originally was made for autistic children, but it illustrates, the problem, and dosing for magnesium. The doctor then decides how many mg of magnesium to apply to the child. It comes in a 4oz jar, or 120grams total. So 1 gram scoop would last 120 days.