Are over-the-counter night guards for tooth grinding less effective than ones from a dentist?

Typically yes: In my opinion, otc mouthguard are a "stopgap" measure until a professionally prescribed device can be made. Most otc guards are the soft "boil and bite" or "one size fits all" types of products. Current science indicates that a "hard" mouth guard device that your dentist can design and prescribe for you are much better to halt the constant trigger mechanism of clenching and bruxing.

Yes: It is important for you to know, if you are a grinder, that over the counter nightguards can actually create TMJ problems. The reason for this is you bite down a lot harder in your sleep than you would being awake. This can put forces on the joint, and dislodge the disc supporting the joint. Short term you can get away with it, but long term (over a month) it can cause damage.

Yes: Otc mouthguards may be useful short term till you can see your dentist. They do not fit as well as a custom made night guard . They also do not last very long since they are usually made of a soft plastic material.

Yes and: They are not only less effective but potentially can cause other problems, in addition to not solving the problem you need one for in the first place. This is not an example of where you can save money. In most cases, it's just throwing out money.

Not really: There are many types of mouthguards for grinding and clenching. Over the counter ones actually may be too thick and throw the muscles into more spasms. Dentists can make custom made, well fitting mouthguards, either upper or lower (i like the lower ones.) the guards can be made of soft plastic or hard acrylic...Depends on severity of grinding. For daytime clenching, other types are indicated.