Yes. Psyllium supplements like Metamucil and Pure for Men are considered safe for people on PrEP.
Psyllium does have the potential to interfere with the absorption of medications and nutrients. Because of that potential, it is a common clinical and pharmacy recommendation that you take psyllium supplements at least 1-2 hours before or after taking other medications. This is a general recommendation, not one specific to PrEP. There is no specific evidence that taking psyllium supplements at the same time as Truvada or Descovy (or their generic equivalents) affects absorption of the drugs, but there is also no evidence that it does not; because there is no evidence either way, some people choose not to take them at the same time. There are no documented cases of PrEP failure associated with psyllium supplements.
There are no known interactions between Truvada or Descovy and psyllium supplements.
The Metamucil FAQ for healthcare providers gives this guidance: “Bulk fibers may affect how well other medications work. If your patients are taking a prescription medication by mouth, they should take Metamucil at least two hours before or two hours after the prescribed medicine.... All oral prescription medications are covered by this label precaution."
The Pure for Men FAQ states: “We normally suggest that if taking Pure for Men while on another medication, just wait an hour or two in between servings of each."
Drugs.com advises: “Avoid taking other oral (by mouth) medicines within 2 hours before or 2 hours after you take psyllium. Bulk-forming laxatives can make it harder for your body to absorb other medications, possibly making them less effective.”
WebMD advises: “This product may decrease the absorption of other medications you may be taking. Take this product at least 2 hours from your other medications. Ask your doctor or pharmacist for more information or if you have any questions.”
Medscape advises: “Separate dose by at least 2 hr from other drug therapies.”
The Harvard Health Letter summarizes: “Fiber goes through the digestive system without getting digested or absorbed. If there's lots of fiber and medication in the intestine at the same time, it's possible that the medication may get swept along with the fiber, so it will be excreted instead of fully absorbed…. So, just to be on the safe side, it might be prudent to take your medications two to three hours before or after your fiber supplement.” - Benedict DiRusso, Harvard University Health Services