Can you use Vaseline as lube?

No, not for vaginal sex and never with latex condoms. Petroleum jelly is on the WHO list of products that damage latex. It is not water soluble, so it stays in the vagina for days, and a published study of US women found petroleum jelly use inside the vagina was associated with bacterial vaginosis.

The condom problem.

The WHO/UNFPA/FHI360 advisory on lubricants names petroleum based products such as Vaseline, alongside mineral oil and cooking oils, as having a highly damaging effect on latex condoms. Latex weakens within minutes of contact. Polyisoprene condoms, the latex free "natural feel" kind, fail the same way. Only polyurethane and nitrile condoms tolerate petroleum jelly, and most people do not own those.

A condom that breaks is a condom that did nothing. If condoms are part of the plan, petroleum jelly is off the table.

The lingering problem.

A water based lube is gone within hours, carried out by the body's own fluids. Petroleum jelly is not water soluble. It coats the tissue and stays until it is physically removed, which inside the vagina means days. During that time it traps whatever is under it, bacteria included, against the lining, and it changes the environment the normal bacteria depend on.

It also does not wash out of fabric. Sheets, underwear and towels keep it.

The study.

In 2013 Brown and colleagues published a study in Obstetrics and Gynecology of 141 women in Los Angeles. Seventeen percent reported using petroleum jelly inside the vagina in the past month. Those women were about twice as likely to test positive for bacterial vaginosis as women who did not. It is one study, it shows an association rather than proof of cause, and it is the best data on the question. Nobody has published a study showing the opposite.

When petroleum jelly is fine.

On the outside. For chafed skin on the thighs or the outer vulva, a thin layer of petroleum jelly does what it was made for: it seals and protects. The line is the opening of the vagina. External and brief, fine. Internal, or with a condom, no.

For anal sex without condoms some people use it because it lasts. It still lingers and it is still hard to clean up. A silicone lube lasts as long, is condom safe and washes off with soap.

What to use instead.

A bottle of real lube costs about what a jar of Vaseline does. For the all rounder, a water based lube in the WHO ideal osmolality band of 380 mOsm/kg or under. Good Clean Love Almost Naked measures 240 to 269 with a pH of 4.7, works with every condom and toy, and rinses away. For the long lasting feel people reach for petroleum jelly to get, a silicone lube like Uberlube does the same job with three ingredients and no residue problem.

If you have already used it, once is usually nothing. Rinse the outside with warm water, skip soap on the inside, and watch for a change in discharge, smell or comfort over the next week. If something shows up and stays, see a clinician. Nothing here is medical advice.