I found something that said it was ... "Once used in association with pentobarbital for anesthesia, but no longer used because of its many side-effects, including venous thrombosis and hemolysis." So... That's off-putting. I also found a PDF on the topic which I had to hand copy this information from... "Mephenesin is a muscle relaxant which acts mainly on the spinal cord but which also has a definite effect on the thalamus, the basal ganglia, and the brain stem. It relaxes hypertonic muscles, without appreciably reducing motor power, and allays fear and tension.
A number of investigators have reported that the drug is effective by the oral route in disorders associated with skeletal muscle spasm. ... Laurence (1952) found that is spastic paralysis, oral administration produced benefit in only two out of 27 cases, but good results have been reported in treatment of spasticity of multiple sclerosis. Mephenesin has also been used successfully in the control of tetanus convulsions, alcoholism, anxiety states, premenstrual tension, and other spastic and hyperkinetic conditions.
The main disadvantages of Mephenesin are its brevity of action and the relatively large doses required to secure a satisfactory response. Animal experiments have indicated that a new derivative Mephenesin carbamate, may largely overcome these disadvantages...
... A PHARMACOLOGICAL NOTE ON MEPHENESIN AND MEPHENESIN CARBAMATE
Mephenesin is absorbed readily from the gastro-intestinal tract and it is conjugated and excreted rapidly. Clinical efficacy is therefore dependant on the administration of adequate doses at regular intervals. It is now generally agreed that the average dosage for an adult is one to three grammes, given four or five times daily. The toxicity of the drug by the oral route is very low, but mild nausea and transient vertigo may occur, especially is the Mephenesin is taken on an empty stomach. In order to minimize these side-effects, it is recommended that "Tolserol" treatment should be commenced with one gramme administered four times daily after meals or after taking a draught of milk or other liquid, and that this dose should be gradually increased until a satisfactory response is obtained."
So... According to that your dose should actually be somewhere from 2 - 6 of those pills to START with assuming you're an adult... Also, the fact that it is rapidly absorbed and excreted would explain why the effects may quickly deteriorate. Finally, as this article states that it has potential benefit of use with alcoholics, this leads me to potentially believe (But not necessarily claim) that this MAY have effect on GABA receptors in the brain (Stating that it may help with "anxiety states" also makes this seem plausible) I can understand how it may give some effects similar to a Benzo or opiate. However, since it's OTC and very little information is available on it... I think it'd probably be best not to try and mess around with it very much for now. If it's new there is no reason to be a guinea pig, if it's old and it's uncommon or little information exists about it, there's PROBABLY A REASON. I think you're better off buying some OTC opiates or Benzos if that's the effect you want. At least they have VOLUMES of information widely available on each, as well as a LONG record of use. Also, when taking one of those two types of drugs you know that you aren't going to try to use for a 2nd or 3rd day in a row and disappointingly get no effects...