Compare

Diaphragm vs Vaginal pH Modulator

A side-by-side comparison of two birth control methods — efficacy, cost, how they work, and who shouldn't use them.

Last verified October 11, 2026Updated October 12, 2026

Side-by-side comparison

Diaphragm A reusable silicone dome that covers the cervix to block sperm, used with spermicide each time you have sex.
Vaginal pH Modulator A prescription gel used before sex that keeps vaginal pH acidic to disable sperm.

Method	Diaphragm A reusable silicone dome that covers the cervix to block sperm, used with spermicide each time you have sex.	Vaginal pH Modulator A prescription gel used before sex that keeps vaginal pH acidic to disable sperm.
Efficacy	Diaphragm 83% typical use Least effective: under 90 out of 100 people successfully prevent pregnancy per year.	Vaginal pH Modulator 86.3% typical use, over 7 menstrual cycles Measured over 7 menstrual cycles, not over a year, so it cannot be compared directly with first-year rates.
Cost	Diaphragm $0–$250 Insurance: Usually	Vaginal pH Modulator $0–$362 per package Insurance: Usually
Duration	Diaphragm Reusable for up to 2 years (replace sooner if damaged)	Vaginal pH Modulator Single use (one dose up to 1 hour before each act of sex)
Reversibility	Diaphragm Reversible	Vaginal pH Modulator Reversible
STI protection	Diaphragm No	Vaginal pH Modulator No
How it works	Diaphragm A reusable silicone dome covers the cervix to block sperm from reaching an egg. Used with spermicide for added effectiveness.	Vaginal pH Modulator A gel containing lactic acid, citric acid, and potassium bitartrate keeps vaginal pH in the normal acidic range even in the presence of semen, which reduces sperm motility.
Prescription	Diaphragm Required Hybrid (Fitting or Ship)	Vaginal pH Modulator Required Telehealth Prescription
Side effects	Diaphragm Increased risk of urinary tract infection Vaginal irritation from spermicide Toxic shock syndrome risk (rare)	Vaginal pH Modulator Vaginal burning or irritation Increased risk of urinary tract infection Rare allergic reaction
Who shouldn't use this	Diaphragm Uterine or cervical anomaly preventing fit Sensitivity to silicone or spermicide Inability to insert or remove reliably History of toxic shock syndrome High risk of HIV exposure (because of the spermicide)	Vaginal pH Modulator Recurrent UTIs or urinary tract abnormalities (FDA label says avoid use) Hypersensitivity to lactic acid, citric acid, or potassium bitartrate (FDA label says avoid use)

Key differences

Vaginal pH Modulator's effectiveness was measured over 7 menstrual cycles, not over a year, so it is not compared with Diaphragm's. How long it lasts: Diaphragm, reusable for up to 2 years (replace sooner if damaged); Vaginal pH Modulator, single use (one dose up to 1 hour before each act of sex). Diaphragm: costs $0–$250. Vaginal pH Modulator: costs $0–$362 per package.