You may be aware that there are several forces of nature that behave in an inverse square relationship. For example, if you move a certain distance away from an object, the gravity between you and that object decreases by the square of the difference. This also holds true for light, sound, radiation, and other forces.
Based on that principle, two researchers named Brody and Light have suggested that the benefit/harm ratio of a given drug in inversely proportional to the amount of pharmaceutical marketing applied. For example, sleeping pills which are not much more effective than an over-the-counter antihistamine, are marketed with forces often reserved for beer and pizza.
In their article suggesting this hypothesis, they go on to detail several marketing strategies which might be utilized at gaining market share, increasing prescription drug use, and changing how people prescribe.
Reducing diagnostic thresholds - Cutoffs to diagnose diabetes have drifted down over the years, but little evidence suggests that aggressively reducing blood sugar improves outcomes. But lower cutoffs might prompt people to prescribe more drugs anyway. My response: definitions of diseases change over the years, sometimes for the better. Homosexuality used to be considered a psychiatric disease.
Surrogate end points - Some studies do not measure death or heart attacks as outcomes, but instead measure cholesterol levels or blood pressure. This makes it easier to get drugs to market (since the studies can me much smaller). Some drugs, which show promise with these outcomes, go on to show no benefit with later studies of important outcomes (see torcetrapib). My response: Sometimes surrogate end points are the only ones suitable for study because finding 10,000 patients to follow over 5 years is tough and expensive.
Exaggerated safety and efficacy claims - This is easy to imagine. Upsell the good stuff and downplay the bad.
Creating new "diseases" - Prehypertension and prediabetes potentially expands the number of people to be treated. If this reduces bad outcomes, this could be good for patients, but even if it does not, more people on medications is good for the bottom line of pharmaceutical companies.
Encouraging unapproved uses - Drug company reps are not allowed to talk about uses for drugs that are not approved by the FDA. They can, however, hire doctors to talk about "off-label" uses. My response: this is true and certainly invites the possibility for bias, but many drugs are used off-label to good effect.
So, is there an inverse benefit law? It is not easy to measure but if someone is looking for a research project you probably could count drug ads and compare that to the relative risk reduction of drugs to measure the relationship, I certainly would not be surprised if someone proved it.