While both medications can be used for the same indication (constipation), they are fundamentally different products.
What Is Metamucil?
Metamucil is known as a fiber laxative. Specifically, it is a psyllium fiber. Psyllium fiber is useful for bulking up the stool (just as our bodies do naturally until we feel the need to defecate) and increasing how fast things move through our digestive tract (i.e. increasing 'motility').
In terms of side effects, psyllium fiber is known cause some gas and bloating to a greater extent than other fiber laxatives such as methylcellulose (brand name Citrucel). However, unlike methylcellulose fiber, psyllium fiber is known to DECREASE cholesterol in patients with elevated cholesterol. Bulk forming fiber laxatives like Metamucil are typically used first line for constipation.
If you take other medications, psyllium fiber should be taken at least 2 hours before or 2 hours after other medications taken by mouth. Also, be sure to drink a good amount of water with your dose, and a glass of water after your dose. It reduces esophageal irritation.
What Is Miralax?
Miralax is the brand name for polyethylene glycol 3350. Polyethylene glycol is known as an osmotic laxative. It increases the fluid in the GI tract by binding water and causes this water to be retained in the stool. Polyethylene glycol is not absorbed into your body, it just moves through your GI tract.
Miralax can be used on a daily basis for the treatment or prophylaxis of constipation, but is also used in higher amounts for bowel prep prior to colonoscopies. Metamucil cannot be used as a bowel prep.
In terms of side effects, it sometimes can cause diarrhea, increase how often you go to the bathroom and flatulence. Typically, doses 17 grams and less don't cause too many problems.
While both products can be taken regularly, fiber generally can be taken daily as a supplement and sufficient amounts can be obtained through the diet. Miralax can be taken daily as well, but usually only for chronic constipation.
Both Metamucil and Miralax are available over the counter.